Nutrient Input in the Chagos Archipelago - the Controlling Mechanisms of Chlorophyll-a Distribution

Abstract

Phytoplankton primary productivity is vital in supporting the marine trophic system, and an understanding of the nutrient sources facilitating this is critical for informing conservation management.

Using a combination of in situ and remotely sensed data, we assess the contributions of different nutrient sources and corresponding productivity in a tropical island ecosystem free from anthropogenic nutrient pollution at varying temporal scales. Beyond the near-shore, annual mean normalised fluorescence line height (nFLH, a proxy for chlorophyll-a concentration) was 27% higher in shallow water (approx. 20m) than deep water (>5000m), demonstrating a chlorophyll-a enhancement in shallow waters. Linear regression revealed a significant (p=<0.001, N=852) relationship where increasing sea surface temperature (SST) was associated with decreasing nFLH. The same relationship was also found between SST anomaly and nFLH anomaly. These relationships were stronger in shallow water (R2=0.39 for SST and nFLH, and 0.18 for the anomaly) than deep water (R2=0.17 for SST and nFLH, and 0.10 for the anomaly), indicating shallower waters are more sensitive to strengthened stratification. Elevated SST is often associated with strengthened stratification, restricting entrainment from below the thermocline, and therefore restricting the movement of nutrients to the surface waters.

In situ CTD measurements revealed a highly stratified environment, with a steep pycnocline. In the mouth of Egmont Atoll, the flood tide was, on average, cooler (by 0.07°C) and more saline (by 0.025 PSU) than the ebb tide (two-sampled t-test, p=<0.001). This represents a change in water properties towards those below the pycnocline, suggesting tidally driven mixing is entraining water from below the surface mixed layer. However, no significant difference was observed in nitrate concentration. Instead, nitrate concentration was associated with salinity. Decreases in salinity were associated with increases in nitrate, following precipitation with an approximately 2-day lag, indicating that precipitation-driven surface runoff is a key nutrient source in waters adjacent to land.

Overall, results suggest that high-frequency, short-term variability in nitrate concentration is driven by precipitation, with longer term productivity variability governed by the strength of stratification, and consequential restriction of entrainment.