Context-dependent effects of sedimentation on top-down and bottom-up control of tropical algae

Background/Question/Methods

Abiotic conditions set the context for top-down and bottom-up control of producer communities. Globally, dominance of tropical reef systems has shifted from coral to algae. While anthropogenic alterations of abiotic processes such as sedimentation have well-documented negative impacts on coral, knowledge of the effect of sedimentation on tropical algae is limited. There is some evidence that sedimentation may prove a benefit through inhibiting top-down herbivory and/or alleviating bottom-up nutrient limitation. However, sediment may also block light and limit nutrient/gas exchange. Thus, it is unknown what direction the net effect may be, and if effects are consistent between algal species or sediment types. We examined the costs and benefits of sedimentation on two genera of tropical algae that are increasing in global abundance, Galaxaura and Padina. Mesocosm experiments tested interactions of added nutrients and sedimentation on growth and tissue nutrient stores. Field experiments examined interacting effects of sediment addition (ambient/removal) and herbivores (caged/uncaged) on growth and tissue nutrients. We conducted the field experiments in two locations in Mo’orea, French Polynesia: an inshore reef flat at the mouth of a bay subject to high flux of terrestrial sediment loads, and a reef flat with carbonate sediment of marine origin. 

Results/Conclusions

Field experiments demonstrated strong but opposite effects of terrestrial sediments on growth of the two algal species, while neither type of sediment limited consumption by herbivores. Biomass accumulation of Galaxaura with ambient sedimentation was 15% higher than removal treatments on the reef with organic-rich terrigenous sediment, but removal of low organic content carbonate sediment had no effect. In contrast, terrestrially-derived sediments reduced growth of Padina by 20%. Mesocosm experiments demonstrated Galaxaura is able to utilize nutrients from terrigenous sediment to support growth at rates equivalent to those after pulsed additions of 20 µM nitrate and 2 µM phosphate; however, effects of nutrients and sediment were not additive. For Padina, mesocosm experiments confirmed the negative effects of sediments identified in the field. While nutrient addition alone increased growth of Padina, sediments always reduced growth, indicating that Padina was unable to utilize sediment nutrients. Our study revealed that increased anthropogenic sources of terrestrial sediments to tropical reefs will shift the relative importance of top-down and bottom-up controls on some species of algae by reducing strong nutrient limitation. As such, increased sedimentation on reefs may shift community dominance toward algal species that are able to capitalize on them as a source of nutrients.