Phytoplankton and episodic suspended sediment loading: Phosphate partitioning and mechanisms for survival

Abstract

Enclosures within a turbid reservoir were used in combination with short-term laboratory experiments to examine some effects of suspended sediment loading on the community structure and phosphate uptake of the phytoplankton. Phytoplankton communities in duplicate field enclosures were treated at 7-d intervals for 8 weeks with a natural, fine-particulate clay in hydrated form, for comparison with species composition and abundances in communities from control enclosures without sediment additions. The phytoplankton community in field enclosures 7 d after final sediment addition was similar to that in controls (without sediment additions). The colonial blue-green Merismopedia punctata was dominant in cell number, and mixotrophic and heterotrophic dinoflagellates were dominant in biomass (based on cell surface area). The surprisingly diverse dinoflagellate assemblage consisted mostly of delicate athecate Gymnodinium spp. which were missed by conventional preservation techniques. Autoradiographs from short-term laboratory assays with trace concentrations of 33PO43− revealed that clay particles adsorbed the radiolabel and also stimulated 33PO43− uptake among all phytoplankton taxa examined. Algal uptake of radiolabeled P was highest immediately after sediment addition, indicating that one mechanism for survival under episodic sediment loading may be rapid uptake of PO43− for subsequent use after adverse conditions. Other apparent mechanisms involved in phytoplankton survival included rapid colony fragmentation (Merismopedia), partial or complete reliance on heterotrophy (dinoflagellates), and rapid formation of temporary cysts (dinoflagellates). Such mechanisms likely facilitate the characteristic resilience of abundant phytoplankton in turbid systems.