Effects of Zooplankton Grazing on Phytoplankton Size-Structure and Biomass in the Lower Hudson River Estuary

• Published in: Estuaries 1996-12, Vol.19 (4), p.874-889
• Main Authors: Lonsdale, Darcy J., Cosper, Elizabeth M., Doall, Michael
• Format: Article
• Language: English
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Biological and medical sciences; Brackish; Brackish water ecosystems; Carbon; Copepoda; Diatoms; Ditylum brightwellii; Egg production; Estuaries; Fundamental and applied biological sciences. Psychology; Ingestion; Phytoplankton; Plankton; Primary productivity; Surface water; Synecology; Thalassiosira pseudonana; Zooplankton
• ISSN: 0160-8347; 1559-2758
• DOI: 10.2307/1352304

The impact of mesozooplankton (>210 μm, mostly adult copepods and late-stage copepodites) and micrometazoa (64-210 μm, mostly copepod nauplii) on phytoplankton size structure and biomass in the lower Hudson River estuary was investigated using various 14 C-labeled algal species as tracers of grazing on natural phytoplankton. During spring and summer, zooplankton grazing pressure, defined as% = mg C ingested m-2 h-1 mg C produced m-2 h-1 (depth-integrated rates) × 100, on total phytoplankton ranged between 0.04% and 1.9% for mesozooplankton and 0.1% and 6.6% for micrometazoa. The greatest grazing impact was measured in fall when 20.2% and 44.6%, respectively, of the total depth-integrated primary production from surface water phytoplankton was grazed. Mesozooplankton exhibited some size-selective grazing on phytoplankton, preferentially grazing the diatom Thalassiosira pseudonana over the larger diatom Ditylum brightwelli, but this was not found for micrometazoa. Neither zooplankton group grazed on the dinoflagellate Amphidinium sp. We conclude that metazoan zooplankton have a minimal role in controlling total phytoplankton biomass in the lower Hudson River estuary. Differences in the growth coefficients of various phytoplankton size-fractions-not grazing selectivity-may be the predominant factor explaining community size-structure.