Influence of hydrology on phytoplankton species composition and life strategies in a subtropical coastal lagoon periodically connected with the Atlantic Ocean

A survey was carried out to investigate the relationship of phytoplankton biovolume, structure, and species life strategies with major abiotic factors in a subtropical choked coastal lagoon (34°33′S, 54°22′W) naturally connecting with the Atlantic Ocean several times a year. Marine and limnetic influence areas were sampled on a monthly basis during two periods, one of low rainfall and high conductivity (August 1996 to February 1998) and a second period with the opposite tendency (December 1998 to March 2000). Photosynthetically active radiation availability was high and reached the bottom (>1% of the incident light), while dissolved inorganic nitrogen (0.6–18.4 μM), soluble reactive phosphorus (<0.3–2.7 μM), and reactive silica (5–386 μM) were highly variable. Life strategies were identified in the phytoplankton as a function of morphology. C-strategists, invasive planktonic and epipelic species of small size, and R-strategists, mixing-dependent species of medium size, characterized this permanently mixed system. High frequency of exchange with the ocean prevented high biomass accumulation. Phytoplankton biomass was lower in the second period of high rainfall (2.3 and 1.1 mm³ 1⁻¹ for period 1 and 2 respectively). A canonical correspondence analysis showed that conductivity, nitrogen, phosphorus, and silica were the main environmental variables explaining phytoplankton species composition patterns. During the first period, Bacillariophyceae (mostly pennate species) and the potentially toxicPrococentrum minimum were dominant; during the second period a higher contribution of flagellates (Cryptophyceae, Euglenophyceae, Prasinophyceae, and flagellates <7 μm) was found. Differences of phytoplankton biomass, main taxonomic groups, and strategies were found between periods but not between limnic and marine areas, suggesting that hydrological dynamic is more relevant than seasonal and spatial differences.