Trophic functioning and nutrient flux in a highly productive tropical lagoon.

Chiku Lagoon is a highly productive tropical lagoon with high fishery yields. Trophic networks and stoichiometrically linked water-salt-nutrient budgets were constructed to relate the functioning of the food web to nonconservative behavior of nutrients in the lagoon. Network analysis showed that the lagoon is more dependent on phytoplankton than detritus and periphyton to generate food sources for consumers. Nevertheless, detritivory is more important than herbivory in the food web. Transfer efficiency is high at low trophic levels, but declines at higher levels due to the high fishery pressure. Thus, only a small fraction of organic matter (15%) is recycled, and this all through detrital pathways, most of which involve only two compartments. Summation of individual rate measurements for primary production and respiration yielded an estimate of +249 g C m year, suggesting an autotrophic ecosystem. An alternative biogeochemical approach demonstrated that the lagoon is a large sink for total dissolved nitrogen and phosphorus, and the net system metabolism was calculated to be +144 g C m year, thus providing a biogeochemical explanation for the high productivity of Chiku Lagoon. Our results suggest that the high fishery yield in Chiku Lagoon can be attributed to high planktonic productivity induced by the high rate of nutrient loading, and the straight-through pathways of the food web.