Seasonal cycles of phytoplankton biomass and primary production in a tropical temporarily open-closed estuarine lagoon — The effect of an extreme climatic event

Temporarily open-closed estuaries and estuarine lagoons are among the most complex aquatic ecosystems, prone to undergo rapid changes in response to global change and other anthropogenic impacts. Nonetheless, studies on the factors that control annual cycles of phytoplanktonic biomass and primary production in such systems, especially tropical ones, are still scarce. Even less information exists on the effect increasingly frequent extreme climatic events (ECE) might have on their dynamics. For this purpose, we monitored the changes in ecological conditions in the Los Micos estuarine lagoon (Honduras) by sampling monthly during an annual cycle that included several changes in the lagoon's mouth phase and attempted to understand which environmental factors affect phytoplanktonic biomass and primary production. We also evaluated the impact of, and recovery from, a tropical storm ECE. Annual mean net production (Pn), integrated for the euphotic zone, (4.3 ± 2.8 gC m⁻² d⁻¹) and Chlorophyll a (27.1 ± 19.1 mg m⁻³) values in Los Micos place it as one of the more productive estuaries worldwide. The physico-chemical characteristics of the lagoon clearly depended on mouth phase; however, the values of Chla and Pn did not show significant differences between the open and closed phases. The application of distance-based multivariate linear models did not show any clear dominant model being able to explain the observed Chla and Pn patterns. The most parsimonious models included among others, salinity, particulate organic carbon and PO₄³⁻, which suggests that primary production is controlled by multiple factors. During the ECE, about 19% of DIN, 91% of DSi and PO₄³⁻, 60% of particulate organic carbon and nitrogen, and 86% of Chla were exported to the sea, greatly reducing Pn. However, Chla and Pn values recovered to pre-storm levels within 30 days, indicating that these biological variables are highly resilient in Los Micos Lagoon.