COS 23-9
Natural nutrient subsidies drive increased variability in benthic community structure and energy flow on coral reefs: Insights from undisturbed reef systems
Ecological studies of community structure, energy flow, competitive dynamics, and nutrient subsidies are often confounded by background anthropogenic disturbances. Remote, uninhabited islands offer rare opportunities to escape such confounding factors and study fundamental ecological processes in the absence of local stressors. The Southern Line Islands (SLI) of Kiribati span a large natural gradient of ambient nutrient availability, which provides a unique natural experiment to study the effects of long-term nutrient enrichment on the structure and function of coral reefs in the absence of humans. Historically, this has proven challenging due to the limited temporal and spatial scales of most nutrient enrichment studies, which do not allow for the communities and processes in question to reach equilibrium and are often cofounded by background nutrient levels. As such, we took advantage of this remote coral reef system to ask: how do nutrient subsidies alter benthic community structure and energy flow across islands and are they structured consistently across depths? At each island, macroalgae and two coral species were collected for stable isotope analysis along with water nutrient samples at 5 m depth intervals from 10-30 m on the leeward reef slope. High-definition video transects were recorded for percent cover of benthic organisms.
Results/Conclusions
Community structure was consistent between the two oligotrophic islands with coral and crustose coralline algae (CCA) comprising ~80% of the benthos and calcified macroalgal cover ranging from 6-15%. As nutrient subsidies became more common in the northern islands, however, variance in community structure increased markedly. At these islands, coral and CCA/calcified macroalgal cover ranged from 19-80% and 3-75%, respectively. On the oligotrophic islands macroalgal cover generally increased with depth, suggesting nutrients as the primary source of energy and light playing a secondary role. Conversely, benthic communities appeared homogenized across depths at enriched islands, suggesting that the presence of higher ambient nutrient levels may cause a breakdown of zonation. Macroalgal pigment concentrations increased as a function of nutrient availability and generally increased with depth. Stable isotope signatures revealed increasing nutrient availability with depth at the oligotrophic islands and a more homogeneous pattern at islands that experience regular nutrient subsidies. These patterns provide insight to the influence of historical nutrient enrichment on coral reefs and suggest 1) long-term nutrient enrichment can alter the community attractor within benthic communities on these islands and 2) that nutrient availability may overwhelm the ubiquitously important depth zonation seen on many coral reefs.