Climate change is altering naturally fluctuating environmental conditions in coastal and estuarin... more Climate change is altering naturally fluctuating environmental conditions in coastal and estuarine ecosystems across the globe. Departures from long-term averages and ranges of environmental variables are increasingly being observed as directional changes [e.g., rising sea levels, sea surface temperatures (SST)] and less predictable periodic cycles (e.g., Atlantic or Pacific decadal oscillations) and extremes (e.g., coastal flooding, marine heatwaves). Quantifying the short- and long-term impacts of climate change on tidal marsh seascape structure and function for nekton is a critical step toward fisheries conservation and management. The multiple stressor framework provides a promising approach for advancing integrative, cross-disciplinary research on tidal marshes and food web dynamics. It can be used to quantify climate change effects on and interactions between coastal oceans (e.g., SST, ocean currents, waves) and watersheds (e.g., precipitation, river flows), tidal marsh geomor...
Ocean acidification is thought to benefit seagrasses because of increased carbon dioxide (CO2) av... more Ocean acidification is thought to benefit seagrasses because of increased carbon dioxide (CO2) availability for photosynthesis. However, in order to truly assess ecological responses, effects of ocean acidification need to be investigated in a variety of coastal environments. We tested the hypothesis that ocean acidification would benefit seagrasses in the northern Gulf of Mexico, where the seagrasses Halodule wrightii and Ruppia maritima coexist in a fluctuating environment. To evaluate if benefits of ocean acidification could alter seagrass bed composition, cores of H. wrightii and R. maritima were placed alone or in combination into aquaria and maintained in an outdoor mesocosm. Half of the aquaria were exposed to either ambient (mean pH of 8.1 ± 0.04 SD on total scale) or high CO2 (mean pH 7.7 ± 0.05 SD on total scale) conditions. After 54 days of experimental exposure, the δ 13 C values were significantly lower in seagrass tissue in the high CO2 condition. This integration of a different carbon source (either: preferential use of CO2, gas from cylinder, or both) indicates that plants were not solely relying on stored energy reserves for growth. Yet, after 41 to 54 days, seagrass morphology, biomass, photo-physiology, metabolism, and carbon and nitrogen content in the high CO2 condition did not differ from those at ambient. There was also no indication of differences in traits between the homo-or hetero-specific beds. Findings support two plausible conclusions: 1) these seagrasses rely heavily on bicarbonate use and growth will not be stimulated by near future acidification conditions or 2) the mesohaline environment limited the beneficial impacts of increased CO2 availability.
The impact of anthropogenic eutrophication on the productivity and metabolism of estuarine sedime... more The impact of anthropogenic eutrophication on the productivity and metabolism of estuarine sediments has received relatively little attention. In this study, we investigated the separate and combined effects of decreased light availability and sediment nutrient enrichment, 2 of the most important impacts of anthropogenic eutrophication, on sediment primary production and metabolism in 2 eutrophic subestuaries of Weeks Bay (Alabama, USA), seasonally, over a year. We found a significant effect of shading on both sediment primary production and metabolism, mostly due to decreased photosynthetic rates of benthic microalgae coupled with simultaneous increases in sediment respiration rates. The ratio of mean production in non-shaded plots to that in shaded plots was close to 1 and tended to occur at natural bottom light intensities <100 µmol m-2 s-1 during the summer experiments, whereas higher light intensities tended to increase the ratio (i.e. higher values under non-shaded conditions). The ratio tended to decrease and become closer to 1 at the highest values of light intensity recorded (> 600 µmol photons m-2 s-1). Nutrient enrichment had little effect on sediment primary production and metabolism in the estuarine sites studied; however, in some instances nutrient enrichment influenced the negative effects of reduced light on sediment net production. The results demonstrate that the impact of light reduction on sediment primary production and metabolism in a turbid, nutrient-rich estuary is greater than that of additional sediment nutrient enrichment.
Humans are modifying the availability of nutrients such as nitrogen (N) and phosphorus (P), and i... more Humans are modifying the availability of nutrients such as nitrogen (N) and phosphorus (P), and it is therefore important to understand how these nutrients, independently or in combination, influence the growth and nutrient content of primary producers. Using meta-analysis of 118 field and laboratory experiments in freshwater, marine and terrestrial ecosystems, we tested hypotheses about co-limitation of N and P by comparing the effects of adding N alone, P alone, and both N and P together on internal N (e.g. %N, C:N) and P (e.g. %P, C:P) concentrations in autotroph communities. In particular, we tested the following predictions. First, if only one nutrient was limiting, addition of that nutrient should decrease the concentration of the other nutrient, but addition of the non-limiting nutrient would have no effect on the internal concentration of the limiting nutrient. If community co-limitation was occurring then addition of either nutrient should result in a decrease in the internal concentration of the other nutrient. Community co-limitation could also result in no change-or even an increase-in N concentrations in response to P addition if P stimulated growth of N fixers. Finally, if biochemically dependent colimitation was occurring, addition of a limiting nutrient would not decrease, and could even increase, the concentration of the other, co-limited nutrient. We found no general evidence for the decrease in the internal concentration of one nutrient due to addition of another nutrient. The one exception to this overall pattern was marine systems, where N addition decreased internal P concentrations. In contrast, P addition increased internal N concentrations across all experiments, consistent with co-limitation. These results have important implications for understanding the roles that N and P play in controlling producer growth and internal nutrient accumulation as well as for managing the effects of nutrient enrichment in ecosystems.
Among potential compensatory mechanisms helping to preserve certain levels of homeostasis, ingest... more Among potential compensatory mechanisms helping to preserve certain levels of homeostasis, ingestion rates (g WW diet per g WW of urchin) were higher for seagrass and macroalgae diets than for the nutrient-rich formulated diet. In contrast, absorption and growth efficiencies displayed significant negative associations with nutrient contents in diets and did not exhibit nutritional compensation. Overall, our results suggest that resource stoichiometry strongly determines the growth rate of individuals (R = 0.88, P < 0.01), and moderate variability in C:N:P ratios of sea urchins possibly arise from differences in the allocation of proteins and RNA to body components, similarly to what has been proposed by the growth rate hypothesis. Communicated by U. Sommer.
There is little information concerning the impacts of urban development within watersheds on nutr... more There is little information concerning the impacts of urban development within watersheds on nutrient budgets in coastal bayous of the North Central Gulf of Mexico. The overall objective of the current study was to detect if nutrient budgets differed among bayous with ...
Many studies show that seagrass d 15 N ratios increase with the amount of urbanization in coastal... more Many studies show that seagrass d 15 N ratios increase with the amount of urbanization in coastal watersheds. However, there is little information on the relationship between urbanization and seagrass d 15 N ratios on a global scale. We performed a meta-analysis on seagrass samples from 79 independent locations to test if seagrass d 15 N ratios correlate with patterns of population density and fertilizer use within a radius of 10-200 km around the sample locations. Our results show that seagrass d 15 N ratios are more influenced by intergeneric and latitudinal differences than the degree of urbanization or the amount of fertilizer used in nearby watersheds. The positive correlation between seagrass d 15 N ratios and latitude hints at an underlying pattern in discrimination or a latitudinal gradient in the 15 N isotopic signature of nitrogen assimilated by the plants. The actual mechanisms responsible for the correlation between d 15 N and latitude remain unknown.
We examined the leaf age dependence, magnitude and variability of herbivore consumption of Posido... more We examined the leaf age dependence, magnitude and variability of herbivore consumption of Posidonia oceanica (L.) Delile leaves in the Spanish Mediterranean Two patterns of herbivore consumption along the lifespan of P. oceanica leaves were found, namely a linear and a parabolic curve of cumulative leaf consumption versus leaf age, which are indicative of leaf-age-independent consumption rates and preference for mid-aged leaves, respectively. The flsh Boops salpa contributed about 75% to the total herbivore consumption, and seemed to be responsible for the parabolic pattern of consumption with leaf age. Herbivory appeared to be a minor factor in the control of P. oceanica production since it only accounted, on the average, for about 2 % of its leaf production. This percentage tends to increase in the northern populations of the Spanish Mediterranean as a result of the longer leaf lifespan and associated higher cumulative consumption values, and the lower leaf production of these populations. It is concluded that, in spite of the low levels of herbivory on P oceanica in the Mediterranean, this seagrass supports a high herbivore production due to its large primary production.
Consumer-diet discrimination factors (Δ 15 N and Δ 13 C) are often applied without corroboration ... more Consumer-diet discrimination factors (Δ 15 N and Δ 13 C) are often applied without corroboration from laboratory experiments. Deviations in Δ 15 N and Δ 13 C that may occur from different diet types were quantified by raising 120 sea urchins Lytechinus variegatus in laboratory tanks on 3 different diets: seagrass Thalassia testudinum, red foliose macroalgae (Grauteloupia sp. and Palmaria palmata) and a mixed diet specifically formulated for L. variegatus. Patterns of δ 13 C and δ 15 N and resulting fractionation factors were then determined in muscle, gonad, gut, test organic matrix and whole individuals. Tissue δ 13 C values showed a strong positive association with δ 13 C of diets and estimates of absorption efficiency (R 2 = 0.81). The seagrass diet consistently resulted in negative Δ 13 C values in all tissues (from −0.86 ‰ in muscle to −1.63 ‰ in gonad) and whole individuals (−1.19 ‰), whereas macroalgal and formulated diets showed positive values (0.11 and 0.19 ‰, respectively). Only individuals on the formulated diet clearly reached isotopic equilibrium for δ 13 C, suggesting that other lower quality diets may have resulted in more continuous reallocation of internal resources. δ 15 N values increased as the nitrogen content of these diets decreased (3.18, 1.21 and 0.82 ‰ for seagrass, macroalgae and formulated diets, respectively). Overall differences in the biochemical composition of diets and a robust relationship between δ 13 C and δ 15 N signatures suggest that protein quantity and quality could be central in driving isotope fractionation. The influence of macrophyte material type in the diet can be stronger than that of trophic level for both δ 13 C and δ 15 N, so further compound-specific isotope analyses are necessary to determine reliable values of Δ 13 C and Δ 15 N for ecological applications.
The incorporation of δ 13 C and δ 18 O from respired CO 2 to tests and whole calcified tissues (W... more The incorporation of δ 13 C and δ 18 O from respired CO 2 to tests and whole calcified tissues (WCT) of the sea urchin Lytechinus variegatus was investigated in 120 individuals to elucidate the influence of diet type on the calcification process. Sea urchins were raised during 4 mo in controlled seawater tanks using 3 different diets (δ 13 C; δ 18 O) (means ± SE): seagrass (-10.2 ± 0.1 ‰; -15.2 ± 0.3 ‰), red macroalgae (-17.8 ± 1.4 ‰; -21.6 ± 0.8 ‰), and a formulated diet (-21.5 ± 0.04 ‰; -25 ± 0.4 ‰). Individuals fed the formulated and red macroalgae diets were depleted in both δ 13 C and δ 18 O compared to those fed seagrass. The isotope composition of skeletons mirrored that of organic carbon (δ 13 C o ) and δ 18 O in diets, but contributions of dietary carbon were higher for urchins fed formulated (40.2 ± 1.2% in test and 34.8 ± 0.4% in WCT) or macroalgae diets (40.38 ± 1.1% in test and 32.1 ± 0.4% in WCT) than for those fed seagrass (29.1 ± 1% in test and 29.9 ± 0.7% in WCT), concurrently with greater growth rates and distinctive rates of fractionation. Differences between test and WCT contributions among diets could not be explained by patterns of biomass allocation across calcified structures (i.e. similar test to spines+lantern ratios for all diets). This suggests that individuals fed red macroalgae or formulated diets increased the amounts of dietary carbon going into their tests, whereas individuals fed seagrass did not. Overall, we identified diet type as a new factor in the process of carbonate deposition that could influence species' responses to changes in ocean chemistry. We suggest that the reconstruction of sea urchin paleodiets might also be possible from fossil records. Test structures may be particularly useful for this purpose, because they accumulate high contributions of metabolic carbon that enhance the detection of differences in the isotopic signatures of diets.
Meadows of the endemic seagrass Posidonia oceanica are threatened in the Mediterranean due to a g... more Meadows of the endemic seagrass Posidonia oceanica are threatened in the Mediterranean due to a general deterioration of the light environment that becomes critical when light irradiance is insufficient to meet the carbon requirements of the system. Here, we conduct a 3 wk, in situ shading experiment (8 levels plus controls) to determine the threshold of irradiance for balanced metabolism in a shallow P. oceanica meadow and further assess the recovery of the system 1 wk later. Reduced light irradiance decreased the net community production of the meadow, which may turn negative (i.e. respiration exceeded gross community primary production) under 338 μE m −2 s −1. Shading throughout the experiment did not appear to cause sustained physiological damage to the system since values of net community production after the cessation of shading were similar to pre-experimental, ambient levels. Sediment acid volatile sulfide pools ranged between 0.002 and 0.058 mol m −2 across shading treatments, and the highest pools were observed in the most shaded sediments. At high light impairment, meristematic cell divisions were low, and carbohydrate content in young rhizomes decreased throughout the experiment. Eight days after the cessation of shading, reduced rhizome carbohydrate stores and elevated sediment sulfide levels still persisted in the previously intensively shaded areas. The present study provides evidence of resistance and resilience of the seagrass Posidonia oceanica to light impairment for short (3 wk) periods of time. Although the compensation irradiance of the system varied by ~2-fold, it provides a quantitative estimate of the irradiance threshold at which seagrass meadows may shift from being coastal carbon sinks to CO 2 sources.
The seasonal growth pattern of the 4 seagrass species occurring in the NW Mediterranean (i.e. Cym... more The seasonal growth pattern of the 4 seagrass species occurring in the NW Mediterranean (i.e. Cymodocea nodosa, Zostera noltii, 2. marina, Posidonla oceanica) was studied in populat~ons growing in the same locality (Cala Jonquet, Girona, NE Spain), and thus experiencing the same seasonal (i.e. temperature and light) forcing, to evaluate the contnbution of species-specific responses to seagrass growth seasonality. C. nodosa, 2. noltij, and Z marina showed comparable growth patterns as indicated by significant correlations of growth across species (cross correlation, r > 0.54, p < 0 05) This result provlded evidence of a similarity in the response of these species to seasonal forcing. The seasonal pattern of P. oceanica resembled that of the other species in shoot weight, shoot elongat~on, and ramet recruitment, whereas it differed in internode weight and rhizome elongation. Despite some similarities in seasonal growth patterns, the patterns were lagged by 1 to 2 mo across species, and the magnitude of seasonal growth fluctuations was species-dependent. Species-specific responses of seagrasses to climate forcing should be related to differences in the capacity of the plants to store resources and to the extent of ramet integration among species, both processes being closely related to plant size. Large seagrasses (e g. oceanica), with thick and long-living rhizomes, should be able to store more photoassimilates and to transport them over longer distances than small plants (e.g. C. nodosa), with thinner and shorter-11v1ng riuzomes. Large species should, therefore, be able to grow more independently of env~ronmental conditions than small ones. Moreover, C. nodosa showed the greatest response to temperature fluctuat~ons whereas 2, marina growth was strongly coupled to seasonal l~g h t conditions, indicating d~fferent plant sensitivity to climate fluctuations among species. This study confirms the great var~ability In seagrass seasonality possible under similar seasonal forcing, and demonstrates that seagrass seasonality has both an extrinsic component, dependent on seasonal forcing of light and temperature, and an intrinsic component. The intnnsic component of seagrass seasonality likely involves a differential capacity of the species to regulate the internal resource economy which may buffer, or amplify, the external seasonal forcing. KEY WORDS: Posidonia oceanica. Cymodocea nodosa. Zostera marina Zostera noltii. Seasonality Module size. Module appearance rate Module growth. Ramet recruitment Flowering
The rates of changes in stocks of organic matter and major nutrients (N and P) durlng colonizatio... more The rates of changes in stocks of organic matter and major nutrients (N and P) durlng colonization and development of seagrass Cymodocea nodosa biomass in a semi-closed estuary. Alfacs Bay, Spain, were studied. The non-vegetated sediments of Alfacs Bay contain large reserves of N and P, but most of these nutrients are not readily available for plant growth as they are tied up in organic matter or mineral fractions within the sediment. Seagrass biomass per unit area increased linearly during the first 5 yr after colonization although the very low tissue concentrations of P in leaves of C. nodosa suggested P limitation. Development of plant biomass was accompdnied by a net accumulation of N at a rate of 3.6 g N m-' yr-' Half of this increase was caused by the build up of plant-bound N in living and dead biomass, while the other half was accounted for by accumulation in other N fractions. P accumulated as plant-bound P, but this fractlon constituted less than 1 of the total P pool. Accumulation of plant-bound P was of the same magnitude as depletion of other P fractions, leaving the pool of total P unaffected by plant development. These results suggest that colonization and development C. nodosa in Alfacs Bay facilitate capture and accumulation of external N within the stands, while P may be mobilized from mineral-bound P reserves within the sediments. KEY WORDS: Nutrient accumulation. Phosphorus. Nitrogen. Seagrasses-Sediments. Estuaries
LVe examined the seasona1it)r and magnitude of the leaf-blade, rh~zome and root biomass and produ... more LVe examined the seasona1it)r and magnitude of the leaf-blade, rh~zome and root biomass and production, along with the fate of leaf-blade production, of the 4 Mediterranean seagrass species Posjdon~a oc.eanlca. Cymodocea nodosa, Zostera noltii and Zostera marina in a protected northern Spanish bay (Cala Jonquet, 42' 18.26' N, 3" 18.11' E) to estimate (1) the annual production consumed by herb~vores or decomposed by detritivores and (2) the production In excess of consumption and firstyear decomposition, which should be dn upper limit of long-term burlal of refractory detritus. The leaf, rhizome and root biomass of the 4 species displayed a clear seasonal pattern (which IS in agreement with past studies), except for that of 2. nolt~i, whlch suggested a r a p~d loss of its productlon either to herbivores or as detritus. 2. marina and P. oceanica were the most productive species, and transferred to consumers (herbivores and detritivores] about twice the production transferred by C. nodosa and 2. noltii. Most of the production of the 4 species was decomposed by detritivores, whlch supports the importance of the detritivore food-web in the use and recycling of seagrass production. Consumption of seagrass leaf productlon by herbivores appeared to be higher for C. nodosa and 2. noltji, the species with the greatest leaf turnover rates, than for Z. marina and P. oceanica. Total heterotrophic use of seagrass product~on (consumption by herbivores and decomposition by detritivores) accounted for more than 80% of seagrass production in the 4 species. Yet, the excess of production not consumed nor decomposed during the first year ranged over 1 order of magnitude from the most (Z. marina and P. oceanica) to the least product~ve species (C. nodosa and 2. noltii) and represented a larger percentage of the production of the former species (9.2 and 16.8% respectively) compared ~vith the latter species (about I S ' % ,). That suggests that 2. marinaand P. oceanica may accumulate larger pools of refractory detritus and that their production is recycled more slowly than that of C. nodosa and 2. noltii. These results show marked differences in the fate of production among the 4 Western Mediterranean seagrass species growing in Cala Jonquet and suggest that differences in the 1.eaf turnover rate could contribute to the explanation of differences in the fate of seagrass product~on, the species with fastergrowing leaves losing a higher perccntage of production to herbivores and recycling most of the residual detntal productlon, therefore stonng relatively small pools of refractory detritus.
The loss of salt-marsh habitat is proceeding at an alarming rate worldwide, resulting in the loss... more The loss of salt-marsh habitat is proceeding at an alarming rate worldwide, resulting in the loss of ecosystem function and reduced exchange with adjacent habitats. The immediate result of shoreline erosion is the loss of fringing vegetation, but it is unclear how, and at what temporal scale, intertidal floral zones respond to shoreline loss. Using a transect-based approach over a 3 yr period, we compared community composition among 3 intertidal salt marshes in coastal Alabama that differ in the intensity of wave-generated disturbance. Each study marsh exhibited net annual rates of shoreline retreat, but differed in its exposure to wave-induced disturbance and erosional state. We also examined whether wave attenuation by oyster break waters reduced annual rates of shoreline erosion, providing a means to test whether the intensity of shoreline erosion affects inter-annual patterns of marsh zonation. Community composition differed across sites, but not between breakwater treatments and site-specific rates of shoreline loss, suggesting that differences in community composition are not due to differences in erosion intensity, but to other contrasts across sites. The cohesive nature of the floral communities at each site suggests that marshes maintain their zonation structure by migrating inland as the shoreline recedes. The inland migration of marsh zones will, however, ultimately be limited by the lack of area available for marsh colonization. The preservation of coastal salt marshes is, therefore, vitally contingent on effective shoreline management strategies. KEY WORDS: Community • Erosion • Flora • Gulf of Mexico • Living shorelines • Salt marsh Resale or republication not permitted without written consent of the publisher FREE REE ACCESS CCESS This authors' personal copy may not be publicly or systematically copied or distributed, or posted on the Open Web, except with written permission of the copyright holder(s). It may be distributed to interested individuals on request.
Journal of Experimental Marine Biology and Ecology, 2010
Cascade of interactions Cavia aperea ENSO Herbivory Rainfall change Spartina densiflora marsh Hem... more Cascade of interactions Cavia aperea ENSO Herbivory Rainfall change Spartina densiflora marsh Hemisphere scale events such as El Niño-Southern Oscillation (ENSO) can alter rainfall regimes worldwide, with important effects on species abundance and distribution. The evidence of ENSO effects on terrestrial communities is, however, restricted to a few ecosystem types. We explored the effects of ENSO episodes on plant/terrestrial-herbivore interactions through changes in the rainfall regime in a southwestern Atlantic salt marsh (Mar Chiquita coastal lagoon, Argentina. 37°40′S, 57°23′W). Surveys showed a positive relationship between winter rainfall and the abundance of the wild guinea pig Cavia aperea. The highest salt marsh abundances of C. aperea were associated with rainy periods during El Niño episodes, and the lowest ones were associated with the driest La Niña episodes. Rainfall was negatively associated with marsh sediment salinity, and experiments revealed that increased salinity reduces growth and increases mortality of cordgrass (Spartina densiflora). Salt increase also causes the highest percentage of dry area in S. densiflora leaves and reduced carbon content, and more salt content and secretion in S. densiflora stems. A factorial experiment in which we manipulated C. aperea presence and salinity along the edges of S. densiflora patches showed that plants can asexually invade unvegetated areas when salinity is reduced and C. aperea is excluded. Conversely, S. densiflora edges retracted when salinity was increased or there was C. aperea herbivory. Changes in nutritional quality of S. densiflora could explain the low herbivory of (and lack of impacts from) C. aperea in plots with high salinity. Thus, plant distribution responds directly to climate oscillations through changes in salt stress, and indirectly, through changes in plant-herbivore interactions. Herbivores respond indirectly to climate oscillations through changes in plant food quality, which suggests that top-down effects increase when bottom-up stressors are relaxed. ENSO events have direct and indirect effects on marsh communities that modulate the relative importance of top-down and bottom-up effects and have a considerable effect on the primary productivity of S. densiflora marshes.
Climate change is altering naturally fluctuating environmental conditions in coastal and estuarin... more Climate change is altering naturally fluctuating environmental conditions in coastal and estuarine ecosystems across the globe. Departures from long-term averages and ranges of environmental variables are increasingly being observed as directional changes [e.g., rising sea levels, sea surface temperatures (SST)] and less predictable periodic cycles (e.g., Atlantic or Pacific decadal oscillations) and extremes (e.g., coastal flooding, marine heatwaves). Quantifying the short- and long-term impacts of climate change on tidal marsh seascape structure and function for nekton is a critical step toward fisheries conservation and management. The multiple stressor framework provides a promising approach for advancing integrative, cross-disciplinary research on tidal marshes and food web dynamics. It can be used to quantify climate change effects on and interactions between coastal oceans (e.g., SST, ocean currents, waves) and watersheds (e.g., precipitation, river flows), tidal marsh geomor...
Ocean acidification is thought to benefit seagrasses because of increased carbon dioxide (CO2) av... more Ocean acidification is thought to benefit seagrasses because of increased carbon dioxide (CO2) availability for photosynthesis. However, in order to truly assess ecological responses, effects of ocean acidification need to be investigated in a variety of coastal environments. We tested the hypothesis that ocean acidification would benefit seagrasses in the northern Gulf of Mexico, where the seagrasses Halodule wrightii and Ruppia maritima coexist in a fluctuating environment. To evaluate if benefits of ocean acidification could alter seagrass bed composition, cores of H. wrightii and R. maritima were placed alone or in combination into aquaria and maintained in an outdoor mesocosm. Half of the aquaria were exposed to either ambient (mean pH of 8.1 ± 0.04 SD on total scale) or high CO2 (mean pH 7.7 ± 0.05 SD on total scale) conditions. After 54 days of experimental exposure, the δ 13 C values were significantly lower in seagrass tissue in the high CO2 condition. This integration of a different carbon source (either: preferential use of CO2, gas from cylinder, or both) indicates that plants were not solely relying on stored energy reserves for growth. Yet, after 41 to 54 days, seagrass morphology, biomass, photo-physiology, metabolism, and carbon and nitrogen content in the high CO2 condition did not differ from those at ambient. There was also no indication of differences in traits between the homo-or hetero-specific beds. Findings support two plausible conclusions: 1) these seagrasses rely heavily on bicarbonate use and growth will not be stimulated by near future acidification conditions or 2) the mesohaline environment limited the beneficial impacts of increased CO2 availability.
The impact of anthropogenic eutrophication on the productivity and metabolism of estuarine sedime... more The impact of anthropogenic eutrophication on the productivity and metabolism of estuarine sediments has received relatively little attention. In this study, we investigated the separate and combined effects of decreased light availability and sediment nutrient enrichment, 2 of the most important impacts of anthropogenic eutrophication, on sediment primary production and metabolism in 2 eutrophic subestuaries of Weeks Bay (Alabama, USA), seasonally, over a year. We found a significant effect of shading on both sediment primary production and metabolism, mostly due to decreased photosynthetic rates of benthic microalgae coupled with simultaneous increases in sediment respiration rates. The ratio of mean production in non-shaded plots to that in shaded plots was close to 1 and tended to occur at natural bottom light intensities <100 µmol m-2 s-1 during the summer experiments, whereas higher light intensities tended to increase the ratio (i.e. higher values under non-shaded conditions). The ratio tended to decrease and become closer to 1 at the highest values of light intensity recorded (> 600 µmol photons m-2 s-1). Nutrient enrichment had little effect on sediment primary production and metabolism in the estuarine sites studied; however, in some instances nutrient enrichment influenced the negative effects of reduced light on sediment net production. The results demonstrate that the impact of light reduction on sediment primary production and metabolism in a turbid, nutrient-rich estuary is greater than that of additional sediment nutrient enrichment.
Humans are modifying the availability of nutrients such as nitrogen (N) and phosphorus (P), and i... more Humans are modifying the availability of nutrients such as nitrogen (N) and phosphorus (P), and it is therefore important to understand how these nutrients, independently or in combination, influence the growth and nutrient content of primary producers. Using meta-analysis of 118 field and laboratory experiments in freshwater, marine and terrestrial ecosystems, we tested hypotheses about co-limitation of N and P by comparing the effects of adding N alone, P alone, and both N and P together on internal N (e.g. %N, C:N) and P (e.g. %P, C:P) concentrations in autotroph communities. In particular, we tested the following predictions. First, if only one nutrient was limiting, addition of that nutrient should decrease the concentration of the other nutrient, but addition of the non-limiting nutrient would have no effect on the internal concentration of the limiting nutrient. If community co-limitation was occurring then addition of either nutrient should result in a decrease in the internal concentration of the other nutrient. Community co-limitation could also result in no change-or even an increase-in N concentrations in response to P addition if P stimulated growth of N fixers. Finally, if biochemically dependent colimitation was occurring, addition of a limiting nutrient would not decrease, and could even increase, the concentration of the other, co-limited nutrient. We found no general evidence for the decrease in the internal concentration of one nutrient due to addition of another nutrient. The one exception to this overall pattern was marine systems, where N addition decreased internal P concentrations. In contrast, P addition increased internal N concentrations across all experiments, consistent with co-limitation. These results have important implications for understanding the roles that N and P play in controlling producer growth and internal nutrient accumulation as well as for managing the effects of nutrient enrichment in ecosystems.
Among potential compensatory mechanisms helping to preserve certain levels of homeostasis, ingest... more Among potential compensatory mechanisms helping to preserve certain levels of homeostasis, ingestion rates (g WW diet per g WW of urchin) were higher for seagrass and macroalgae diets than for the nutrient-rich formulated diet. In contrast, absorption and growth efficiencies displayed significant negative associations with nutrient contents in diets and did not exhibit nutritional compensation. Overall, our results suggest that resource stoichiometry strongly determines the growth rate of individuals (R = 0.88, P < 0.01), and moderate variability in C:N:P ratios of sea urchins possibly arise from differences in the allocation of proteins and RNA to body components, similarly to what has been proposed by the growth rate hypothesis. Communicated by U. Sommer.
There is little information concerning the impacts of urban development within watersheds on nutr... more There is little information concerning the impacts of urban development within watersheds on nutrient budgets in coastal bayous of the North Central Gulf of Mexico. The overall objective of the current study was to detect if nutrient budgets differed among bayous with ...
Many studies show that seagrass d 15 N ratios increase with the amount of urbanization in coastal... more Many studies show that seagrass d 15 N ratios increase with the amount of urbanization in coastal watersheds. However, there is little information on the relationship between urbanization and seagrass d 15 N ratios on a global scale. We performed a meta-analysis on seagrass samples from 79 independent locations to test if seagrass d 15 N ratios correlate with patterns of population density and fertilizer use within a radius of 10-200 km around the sample locations. Our results show that seagrass d 15 N ratios are more influenced by intergeneric and latitudinal differences than the degree of urbanization or the amount of fertilizer used in nearby watersheds. The positive correlation between seagrass d 15 N ratios and latitude hints at an underlying pattern in discrimination or a latitudinal gradient in the 15 N isotopic signature of nitrogen assimilated by the plants. The actual mechanisms responsible for the correlation between d 15 N and latitude remain unknown.
We examined the leaf age dependence, magnitude and variability of herbivore consumption of Posido... more We examined the leaf age dependence, magnitude and variability of herbivore consumption of Posidonia oceanica (L.) Delile leaves in the Spanish Mediterranean Two patterns of herbivore consumption along the lifespan of P. oceanica leaves were found, namely a linear and a parabolic curve of cumulative leaf consumption versus leaf age, which are indicative of leaf-age-independent consumption rates and preference for mid-aged leaves, respectively. The flsh Boops salpa contributed about 75% to the total herbivore consumption, and seemed to be responsible for the parabolic pattern of consumption with leaf age. Herbivory appeared to be a minor factor in the control of P. oceanica production since it only accounted, on the average, for about 2 % of its leaf production. This percentage tends to increase in the northern populations of the Spanish Mediterranean as a result of the longer leaf lifespan and associated higher cumulative consumption values, and the lower leaf production of these populations. It is concluded that, in spite of the low levels of herbivory on P oceanica in the Mediterranean, this seagrass supports a high herbivore production due to its large primary production.
Consumer-diet discrimination factors (Δ 15 N and Δ 13 C) are often applied without corroboration ... more Consumer-diet discrimination factors (Δ 15 N and Δ 13 C) are often applied without corroboration from laboratory experiments. Deviations in Δ 15 N and Δ 13 C that may occur from different diet types were quantified by raising 120 sea urchins Lytechinus variegatus in laboratory tanks on 3 different diets: seagrass Thalassia testudinum, red foliose macroalgae (Grauteloupia sp. and Palmaria palmata) and a mixed diet specifically formulated for L. variegatus. Patterns of δ 13 C and δ 15 N and resulting fractionation factors were then determined in muscle, gonad, gut, test organic matrix and whole individuals. Tissue δ 13 C values showed a strong positive association with δ 13 C of diets and estimates of absorption efficiency (R 2 = 0.81). The seagrass diet consistently resulted in negative Δ 13 C values in all tissues (from −0.86 ‰ in muscle to −1.63 ‰ in gonad) and whole individuals (−1.19 ‰), whereas macroalgal and formulated diets showed positive values (0.11 and 0.19 ‰, respectively). Only individuals on the formulated diet clearly reached isotopic equilibrium for δ 13 C, suggesting that other lower quality diets may have resulted in more continuous reallocation of internal resources. δ 15 N values increased as the nitrogen content of these diets decreased (3.18, 1.21 and 0.82 ‰ for seagrass, macroalgae and formulated diets, respectively). Overall differences in the biochemical composition of diets and a robust relationship between δ 13 C and δ 15 N signatures suggest that protein quantity and quality could be central in driving isotope fractionation. The influence of macrophyte material type in the diet can be stronger than that of trophic level for both δ 13 C and δ 15 N, so further compound-specific isotope analyses are necessary to determine reliable values of Δ 13 C and Δ 15 N for ecological applications.
The incorporation of δ 13 C and δ 18 O from respired CO 2 to tests and whole calcified tissues (W... more The incorporation of δ 13 C and δ 18 O from respired CO 2 to tests and whole calcified tissues (WCT) of the sea urchin Lytechinus variegatus was investigated in 120 individuals to elucidate the influence of diet type on the calcification process. Sea urchins were raised during 4 mo in controlled seawater tanks using 3 different diets (δ 13 C; δ 18 O) (means ± SE): seagrass (-10.2 ± 0.1 ‰; -15.2 ± 0.3 ‰), red macroalgae (-17.8 ± 1.4 ‰; -21.6 ± 0.8 ‰), and a formulated diet (-21.5 ± 0.04 ‰; -25 ± 0.4 ‰). Individuals fed the formulated and red macroalgae diets were depleted in both δ 13 C and δ 18 O compared to those fed seagrass. The isotope composition of skeletons mirrored that of organic carbon (δ 13 C o ) and δ 18 O in diets, but contributions of dietary carbon were higher for urchins fed formulated (40.2 ± 1.2% in test and 34.8 ± 0.4% in WCT) or macroalgae diets (40.38 ± 1.1% in test and 32.1 ± 0.4% in WCT) than for those fed seagrass (29.1 ± 1% in test and 29.9 ± 0.7% in WCT), concurrently with greater growth rates and distinctive rates of fractionation. Differences between test and WCT contributions among diets could not be explained by patterns of biomass allocation across calcified structures (i.e. similar test to spines+lantern ratios for all diets). This suggests that individuals fed red macroalgae or formulated diets increased the amounts of dietary carbon going into their tests, whereas individuals fed seagrass did not. Overall, we identified diet type as a new factor in the process of carbonate deposition that could influence species' responses to changes in ocean chemistry. We suggest that the reconstruction of sea urchin paleodiets might also be possible from fossil records. Test structures may be particularly useful for this purpose, because they accumulate high contributions of metabolic carbon that enhance the detection of differences in the isotopic signatures of diets.
Meadows of the endemic seagrass Posidonia oceanica are threatened in the Mediterranean due to a g... more Meadows of the endemic seagrass Posidonia oceanica are threatened in the Mediterranean due to a general deterioration of the light environment that becomes critical when light irradiance is insufficient to meet the carbon requirements of the system. Here, we conduct a 3 wk, in situ shading experiment (8 levels plus controls) to determine the threshold of irradiance for balanced metabolism in a shallow P. oceanica meadow and further assess the recovery of the system 1 wk later. Reduced light irradiance decreased the net community production of the meadow, which may turn negative (i.e. respiration exceeded gross community primary production) under 338 μE m −2 s −1. Shading throughout the experiment did not appear to cause sustained physiological damage to the system since values of net community production after the cessation of shading were similar to pre-experimental, ambient levels. Sediment acid volatile sulfide pools ranged between 0.002 and 0.058 mol m −2 across shading treatments, and the highest pools were observed in the most shaded sediments. At high light impairment, meristematic cell divisions were low, and carbohydrate content in young rhizomes decreased throughout the experiment. Eight days after the cessation of shading, reduced rhizome carbohydrate stores and elevated sediment sulfide levels still persisted in the previously intensively shaded areas. The present study provides evidence of resistance and resilience of the seagrass Posidonia oceanica to light impairment for short (3 wk) periods of time. Although the compensation irradiance of the system varied by ~2-fold, it provides a quantitative estimate of the irradiance threshold at which seagrass meadows may shift from being coastal carbon sinks to CO 2 sources.
The seasonal growth pattern of the 4 seagrass species occurring in the NW Mediterranean (i.e. Cym... more The seasonal growth pattern of the 4 seagrass species occurring in the NW Mediterranean (i.e. Cymodocea nodosa, Zostera noltii, 2. marina, Posidonla oceanica) was studied in populat~ons growing in the same locality (Cala Jonquet, Girona, NE Spain), and thus experiencing the same seasonal (i.e. temperature and light) forcing, to evaluate the contnbution of species-specific responses to seagrass growth seasonality. C. nodosa, 2. noltij, and Z marina showed comparable growth patterns as indicated by significant correlations of growth across species (cross correlation, r > 0.54, p < 0 05) This result provlded evidence of a similarity in the response of these species to seasonal forcing. The seasonal pattern of P. oceanica resembled that of the other species in shoot weight, shoot elongat~on, and ramet recruitment, whereas it differed in internode weight and rhizome elongation. Despite some similarities in seasonal growth patterns, the patterns were lagged by 1 to 2 mo across species, and the magnitude of seasonal growth fluctuations was species-dependent. Species-specific responses of seagrasses to climate forcing should be related to differences in the capacity of the plants to store resources and to the extent of ramet integration among species, both processes being closely related to plant size. Large seagrasses (e g. oceanica), with thick and long-living rhizomes, should be able to store more photoassimilates and to transport them over longer distances than small plants (e.g. C. nodosa), with thinner and shorter-11v1ng riuzomes. Large species should, therefore, be able to grow more independently of env~ronmental conditions than small ones. Moreover, C. nodosa showed the greatest response to temperature fluctuat~ons whereas 2, marina growth was strongly coupled to seasonal l~g h t conditions, indicating d~fferent plant sensitivity to climate fluctuations among species. This study confirms the great var~ability In seagrass seasonality possible under similar seasonal forcing, and demonstrates that seagrass seasonality has both an extrinsic component, dependent on seasonal forcing of light and temperature, and an intrinsic component. The intnnsic component of seagrass seasonality likely involves a differential capacity of the species to regulate the internal resource economy which may buffer, or amplify, the external seasonal forcing. KEY WORDS: Posidonia oceanica. Cymodocea nodosa. Zostera marina Zostera noltii. Seasonality Module size. Module appearance rate Module growth. Ramet recruitment Flowering
The rates of changes in stocks of organic matter and major nutrients (N and P) durlng colonizatio... more The rates of changes in stocks of organic matter and major nutrients (N and P) durlng colonization and development of seagrass Cymodocea nodosa biomass in a semi-closed estuary. Alfacs Bay, Spain, were studied. The non-vegetated sediments of Alfacs Bay contain large reserves of N and P, but most of these nutrients are not readily available for plant growth as they are tied up in organic matter or mineral fractions within the sediment. Seagrass biomass per unit area increased linearly during the first 5 yr after colonization although the very low tissue concentrations of P in leaves of C. nodosa suggested P limitation. Development of plant biomass was accompdnied by a net accumulation of N at a rate of 3.6 g N m-' yr-' Half of this increase was caused by the build up of plant-bound N in living and dead biomass, while the other half was accounted for by accumulation in other N fractions. P accumulated as plant-bound P, but this fractlon constituted less than 1 of the total P pool. Accumulation of plant-bound P was of the same magnitude as depletion of other P fractions, leaving the pool of total P unaffected by plant development. These results suggest that colonization and development C. nodosa in Alfacs Bay facilitate capture and accumulation of external N within the stands, while P may be mobilized from mineral-bound P reserves within the sediments. KEY WORDS: Nutrient accumulation. Phosphorus. Nitrogen. Seagrasses-Sediments. Estuaries
LVe examined the seasona1it)r and magnitude of the leaf-blade, rh~zome and root biomass and produ... more LVe examined the seasona1it)r and magnitude of the leaf-blade, rh~zome and root biomass and production, along with the fate of leaf-blade production, of the 4 Mediterranean seagrass species Posjdon~a oc.eanlca. Cymodocea nodosa, Zostera noltii and Zostera marina in a protected northern Spanish bay (Cala Jonquet, 42' 18.26' N, 3" 18.11' E) to estimate (1) the annual production consumed by herb~vores or decomposed by detritivores and (2) the production In excess of consumption and firstyear decomposition, which should be dn upper limit of long-term burlal of refractory detritus. The leaf, rhizome and root biomass of the 4 species displayed a clear seasonal pattern (which IS in agreement with past studies), except for that of 2. nolt~i, whlch suggested a r a p~d loss of its productlon either to herbivores or as detritus. 2. marina and P. oceanica were the most productive species, and transferred to consumers (herbivores and detritivores] about twice the production transferred by C. nodosa and 2. noltii. Most of the production of the 4 species was decomposed by detritivores, whlch supports the importance of the detritivore food-web in the use and recycling of seagrass production. Consumption of seagrass leaf productlon by herbivores appeared to be higher for C. nodosa and 2. noltji, the species with the greatest leaf turnover rates, than for Z. marina and P. oceanica. Total heterotrophic use of seagrass product~on (consumption by herbivores and decomposition by detritivores) accounted for more than 80% of seagrass production in the 4 species. Yet, the excess of production not consumed nor decomposed during the first year ranged over 1 order of magnitude from the most (Z. marina and P. oceanica) to the least product~ve species (C. nodosa and 2. noltii) and represented a larger percentage of the production of the former species (9.2 and 16.8% respectively) compared ~vith the latter species (about I S ' % ,). That suggests that 2. marinaand P. oceanica may accumulate larger pools of refractory detritus and that their production is recycled more slowly than that of C. nodosa and 2. noltii. These results show marked differences in the fate of production among the 4 Western Mediterranean seagrass species growing in Cala Jonquet and suggest that differences in the 1.eaf turnover rate could contribute to the explanation of differences in the fate of seagrass product~on, the species with fastergrowing leaves losing a higher perccntage of production to herbivores and recycling most of the residual detntal productlon, therefore stonng relatively small pools of refractory detritus.
The loss of salt-marsh habitat is proceeding at an alarming rate worldwide, resulting in the loss... more The loss of salt-marsh habitat is proceeding at an alarming rate worldwide, resulting in the loss of ecosystem function and reduced exchange with adjacent habitats. The immediate result of shoreline erosion is the loss of fringing vegetation, but it is unclear how, and at what temporal scale, intertidal floral zones respond to shoreline loss. Using a transect-based approach over a 3 yr period, we compared community composition among 3 intertidal salt marshes in coastal Alabama that differ in the intensity of wave-generated disturbance. Each study marsh exhibited net annual rates of shoreline retreat, but differed in its exposure to wave-induced disturbance and erosional state. We also examined whether wave attenuation by oyster break waters reduced annual rates of shoreline erosion, providing a means to test whether the intensity of shoreline erosion affects inter-annual patterns of marsh zonation. Community composition differed across sites, but not between breakwater treatments and site-specific rates of shoreline loss, suggesting that differences in community composition are not due to differences in erosion intensity, but to other contrasts across sites. The cohesive nature of the floral communities at each site suggests that marshes maintain their zonation structure by migrating inland as the shoreline recedes. The inland migration of marsh zones will, however, ultimately be limited by the lack of area available for marsh colonization. The preservation of coastal salt marshes is, therefore, vitally contingent on effective shoreline management strategies. KEY WORDS: Community • Erosion • Flora • Gulf of Mexico • Living shorelines • Salt marsh Resale or republication not permitted without written consent of the publisher FREE REE ACCESS CCESS This authors' personal copy may not be publicly or systematically copied or distributed, or posted on the Open Web, except with written permission of the copyright holder(s). It may be distributed to interested individuals on request.
Journal of Experimental Marine Biology and Ecology, 2010
Cascade of interactions Cavia aperea ENSO Herbivory Rainfall change Spartina densiflora marsh Hem... more Cascade of interactions Cavia aperea ENSO Herbivory Rainfall change Spartina densiflora marsh Hemisphere scale events such as El Niño-Southern Oscillation (ENSO) can alter rainfall regimes worldwide, with important effects on species abundance and distribution. The evidence of ENSO effects on terrestrial communities is, however, restricted to a few ecosystem types. We explored the effects of ENSO episodes on plant/terrestrial-herbivore interactions through changes in the rainfall regime in a southwestern Atlantic salt marsh (Mar Chiquita coastal lagoon, Argentina. 37°40′S, 57°23′W). Surveys showed a positive relationship between winter rainfall and the abundance of the wild guinea pig Cavia aperea. The highest salt marsh abundances of C. aperea were associated with rainy periods during El Niño episodes, and the lowest ones were associated with the driest La Niña episodes. Rainfall was negatively associated with marsh sediment salinity, and experiments revealed that increased salinity reduces growth and increases mortality of cordgrass (Spartina densiflora). Salt increase also causes the highest percentage of dry area in S. densiflora leaves and reduced carbon content, and more salt content and secretion in S. densiflora stems. A factorial experiment in which we manipulated C. aperea presence and salinity along the edges of S. densiflora patches showed that plants can asexually invade unvegetated areas when salinity is reduced and C. aperea is excluded. Conversely, S. densiflora edges retracted when salinity was increased or there was C. aperea herbivory. Changes in nutritional quality of S. densiflora could explain the low herbivory of (and lack of impacts from) C. aperea in plots with high salinity. Thus, plant distribution responds directly to climate oscillations through changes in salt stress, and indirectly, through changes in plant-herbivore interactions. Herbivores respond indirectly to climate oscillations through changes in plant food quality, which suggests that top-down effects increase when bottom-up stressors are relaxed. ENSO events have direct and indirect effects on marsh communities that modulate the relative importance of top-down and bottom-up effects and have a considerable effect on the primary productivity of S. densiflora marshes.
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