# Seasonal bottom-up and top-down control of plankton in a hypereutrophic macrotidal lagoon on Brazil’s equatorial coast

## TL;DR

This study examines how seasonal changes affect plankton in a nutrient-rich lagoon in Brazil, finding that both nutrients and grazing influence plankton populations differently in rainy and dry seasons.

## Contribution

The study identifies seasonal shifts in bottom-up and top-down controls on plankton in a tropical lagoon, offering specific management recommendations for eutrophication control.

## Key findings

- Rainy season plankton communities are linked to higher dissolved oxygen and ammonium, while dry season communities correlate with higher salinity and turbidity.
- Phytoplankton density increases with DIP but decreases with dissolved oxygen and grazing by the rotifer Filinia longiseta.
- Microcystis species show frequent peaks, indicating eutrophication risk, though bloom thresholds are not exceeded.

## Abstract

Tropical coastal lagoons often show pronounced seasonal forcing that modulates nutrient supply, light climate, and grazer pressure. We surveyed hypereutrophic Jansen Lagoon (São Luís Island, Brazil) in four campaigns during 2017 (rainy: March and April; dry: September and November) at nine near-surface stations sampled on ebb tide. Phytoplankton and zooplankton were collected and analyzed together with chlorophyll-a and nutrients to assess short-term community responses. Clustering and nMDS revealed clear rainy–dry segregation of communities, and dbRDA linked dry-season samples to higher salinity, turbidity, TP, and silicate, whereas rainy-season samples were associated with higher dissolved oxygen, Secchi depth, ammonium, and DIN. Generalized linear models explained 65% of phytoplankton variance: density increased with DIP and decreased with dissolved oxygen and with the rotifer Filinia longiseta, indicating concurrent bottom-up (nutrients, light/renewal) and top-down (grazing) controls. Microcystis wesenbergii and M. aeruginosa exhibited frequent peaks, underscoring eutrophic risk, though values remained below the bloom threshold applied here. Overall, bottom-up control predominated in the rainy season, whereas grazer pressure intensified in the dry season. Management should couple nutrient-load reductions with measures that shorten residence time, reduce resuspension, and restore macrophytes, with priority to urban margins and semi-enclosed embayments; routine tracking of DO percent saturation, DIN/DIP, Secchi depth, and chlorophyll-a is recommended for long-term assessment.