Trophic functioning and nutrient flux in a highly productive tropical lagoon

Chiku Lagoon is a highly productive tropical lagoon with high fishery yield s. Trophic networks and stoichiometrically linked water-salt-nutrient budge ts were constructed to relate the functioning of the food web to nonconserv ative 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 impo rtant than herbivory in the food web. Transfer efficiency is high at low tr ophic levels, but declines at higher levels due to the high fishery pressur e. Thus, only a small fraction of organic matter (15%) is recycled, and thi s all through detrital pathways, most of which involve only two compartment s. Summation of individual rate measurements for primary production and res piration yielded an estimate of +249 g C m(-2) year(-1), suggesting an auto trophic ecosystem. An alternative bio-geochemical approach demonstrated tha t the lagoon is a large sink for total dissolved nitrogen and phosphorus, a nd the net system metabolism was calculated to be +144 g C m(-2) year(-1), thus providing a biogeochemical explanation for the high productivity of Ch iku 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.