Effects of food web structure and resource subsidies on the patterns and mechanisms of temporal coherence in a tropical coastal lagoon: an experimental mesocosm approach

AIM: The study of the patterns and mechanisms of temporal coherence of ecological variables among lakes has become an important area of limnology. However, no study to date has experimentally tested whether and how resource subsidies and food web configuration affect the patterns and mechanisms of temporal coherence of limnological variables. We conducted a field mesocosm experiment to test the following hypotheses: (i) nutrient enrichment would reduce the temporal coherence of system variables; (ii) fish predation would enhance the temporal coherence of system variables; and (iii) the strength of temporal coherence decreases from physical (water transparency), to chemical (dissolved oxygen concentration [DO]) to biological variables (total zooplankton biomass). METHODS: For 11 weeks, we manipulated fish presence and nutrient (N and P) concentration in a 2 × 2 factorial design in sixteen within-lake enclosures installed in a tropical coastal lagoon. Coherence was estimated by pair-to-pair Pearson's moment correlations of the temporal trajectories of each response variable among enclosures of the same treatment. RESULTS: Fish presence only enhanced the temporal coherence of zooplankton biomass, whereas contrary to our expectations, nutrient addition enhanced the temporal coherence of [DO]. The strength of the individual effects of fish and nutrients on temporal coherence was affected by variable identity, but this variation did not occur in a consistent pattern across variables. However, the interactive effects of fish and nutrients on the temporal coherence of the three variables monitored were not statistically significant. CONCLUSIONS: Our results indicate that local factors, such as fish presence and nutrient availability, may affect the temporal coherence of several system variables, but these effects are better predicted by the strength of direct interactions between the local factor and the variable than by the identity of the variable itself. We conclude that eutrophication and overfishing may alter the coupling of spatial and temporal dynamics of some ecosystem variables