Aquaculture and Fisheries Group

To study the effect of water flow on coral growth, four series of ten coral nubbins of Galaxea fascicularis were exposed to four different flow regimes (0, 10, 20, and 25 cm s -1 , bidirectional flow) for 42 weeks. Buoyant weight, surface area, and polyp number were measured at regular intervals. Net photosynthesis and dark respiration were measured at the corresponding flow speeds, and daily amount of photosynthetic carbon left for coral growth was calculated. Finally, skeletal density and CN content, chlorophyll concentration and dry weight of coral tissue were determined for each coral. Specific growth rate (in day -1 ) decreased with time in each flow treatment. Absence of flow resulted in significantly lower growth rates. Average specific growth rate calculated over the entire experiment was not significantly different between 10 and 20 cm s -1 , while it was significantly higher at 25 cm s -1 . From 10 to 25 cm s -1 , average net photosynthetic rate decreased and average dark respiration rate did not change significantly. Scope for growth based on phototrophic carbon decreased with increasing flow. Growth was not positively correlated with either photosynthesis or respiration, or scope for growth. It is suggested that higher flow rates reduce the chance of disturbance of coral growth by competing algae or cyanobacteria, allowing corals to grow more readily with the maximum specific growth rate possible under the given environmental conditions. Notably, other effects of increased flow, such as increased respiratory rates and increased (in)organic nutrient uptake, might have been equally responsible for the increased growth of the corals in 25 cm s -1 .

The relation between irradiance, skeletal growth and net photosynthesis was studied for the scleractinian coral Galaxea fascicularis to provide experimental evidence for mediation of light-enhanced calcification through photosynthesis. The hypothesis was tested that skeletal growth and photosynthesis are linearly correlated. A long-term experiment was performed in a closed-circuit aquarium system, in which four series of nine nubbins (single polyp clones of a coral colony) of Galaxea fascicularis were exposed to four light treatments (10L:14D): 144 W T8 fluorescent lighting providing an irradiance of 68 µE/m 2 /s and 70, 250 and 400 W Metal Halide lighting providing an irradiance of 38 µE/m 2 /s, 166 µE/m 2 /s and 410 µE/m 2 /s, respectively. Growth of these nubbins was measured as buoyant weight at different time intervals in a 294 day experiment. A lightsaturation curve for photosynthesis was measured in a respirometric flow cell using a 54 week Galaxea fascicularis colony grown at 60 µE/m 2 /s. No saturation of net photosynthesis of Galaxea fascicularis was found at the irradiances tested. The specific growth rate (µ, in day -1 ) of the coral nubbins increased with irradiance. Whereas irradiance varied 11-fold (38 to 410 µE/m 2 /s), buoyant weight (increase after 294 days) increased 5.7 times (2243 to 12374 mg), specific growth rate (1-294 days) increased 1.6 times (0.0103 to 0.0161 day -1 ), while net photosynthetic rate increased 8.9 times (0.009 µmol O 2 /min/cm 2 to 0.077 µmol O 2 /min/cm 2 ). The increase of specific growth rate with irradiance was less than expected based on the increase in net photosynthetic rate with irradiance. This discrepancy between potential energy produced in photosynthesis and energy used for skeletal growth indicates that skeletal growth is not limited by photosynthetic potential at high irradiance levels.

In this study, we tested the hypothesis that the importance of water flow for skeletal growth (rate) becomes higher with increasing irradiance levels (i.e. a synergistic effect) and that such effect is mediated by a water flow modulated effect on net photosynthesis. Four series of nine nubbins of G. fascicularis were grown at either high (600 lE m -2 s -1 ) or intermediate (300 lE m -2 s -1 ) irradiance in combination with either high (15-25 cm s -1 ) or low (5-10 cm s -1 ) flow. Growth was measured as buoyant weight and surface area. Photosynthetic rates were measured at each coral's specific experimental irradiance and flow speed. Additionally, the instantaneous effect of water flow on net photosynthetic rate was determined in short-term incubations in a respirometric flowcell. A significant interaction was found between irradiance and water flow for the increase in buoyant weight, the increase in surface area, and specific skeletal growth rate, indicating that flow velocity becomes more important for coral growth with increasing irradiance levels. Enhancement of coral growth with increasing water flow can be explained by increased net photosynthetic rates. Additionally, the need for costly photo-protective mechanisms at low flow regimes could explain the differences in growth with flow. Communicated by R. H. Richmond. M. Schutter Á J. Verreth Á R

Light is one of the most important abiotic factors influencing the (skeletal) growth of scleractinian corals. Light stimulates coral growth by the process of light-enhanced calcification, which is mediated by zooxanthellar photosynthesis. However, the quantity of light that is available for daily coral growth is not only determined by light intensity (i.e. irradiance), but also by photoperiod (i.e. the light duration time). Understanding and optimizing conditions for coral growth is essential for sustainable coral aquaculture. Therefore, in this study, the question was explored whether more light (i.e. more photons), presented either as irradiance or as light duration, would result in more growth. A series of nine genetically identical coral colonies of Galaxea fascicularis L. were cultured for a period of 18 weeks at different light duration times 8 hours 150 mE m 22 s 21 :16 hours dark, 12 hours 150 mE m 22 s 21 :12 hours dark, 16 hours 150 mE m 22 s 21 :8 hours dark, 24 hours 150 mE m 22 s 21 :0 hours dark) and different irradiance levels (8 hours 150 mE m 22 s 21 :16 hours dark, 8 hours 225 mE m 22 s 21 :16 hours dark and 8 hours 300 mE m 22 s 21 :16 hours dark). Growth was determined every two weeks by measuring buoyant weight. Temperature, salinity and feeding levels were kept constant during the experiment. To detect possible acclimation of the corals to an increased light duration, rates of net photosynthesis and dark respiration were measured, hereby comparing coral colonies grown under an 8:16 hours light (150 mE m 22 s 21 ):dark cycle with corals grown under a 16:8 hours light (150 mE m 22 s 21 ):dark cycle. No increase in growth was detected with either increasing photoperiod or irradiance. Continuous lighting (24 hours 150 mE m 22 s 21 :0 hours dark) resulted in immediate bleaching and the corals died after 14