Abstract

Harmful algal blooms (HABs) are increasing in frequency and severity, underscoring the importance of understanding potential top-down controls of blooms. In the Indian River Lagoon (IRL), a shallow subtropical estuary, one harmful bloom was co-dominated by an unresolved prochlorophyte in the Family Prochlorothricaceae, which reached densities of > 106 cells ml–1 engendering the term “Superbloom.” Experiments were conducted to evaluate grazing rates and the potential for top-down control by an abundant herbivorous copepod, Parvocalanus crassirostris, on the prochlorophyte. Those grazing rates were lower than the rates on a palatable alternative algal food, Isochrysis galbana, when both algal species were presented in monocultures with identical densities. Grazing on the prochlorophyte decreased or ceased at densities over 4.8 × 105 cells ml–1. When the prochlorophyte and the palatable alternative each comprised half of the total density, both species were consumed, but grazing on I. galbana was reduced compared to the grazing rates in a monoculture of this species, especially at higher cell densities. Copepod mortality was observed in treatments with high concentrations of the prochlorophyte, and these treatments contained mucilage. Experiments simulating viscosities produced by prochlorophyte mucilage yielded results consistent with the original grazing experiments (i.e., copepods showed lower grazing rates and higher mortality rates in higher viscosity treatments). Results reveal potential limitations of top-down controls by this grazer on prochlorophyte blooms and HABs that produce mucilage.

Highlights

  • Harmful algal blooms (HABs) are increasing in estuaries around the world (Smayda, 2008; Berry et al, 2015; Phlips et al, 2015; Cao et al, 2017)

  • This study aims to (1) determine the grazing rates of P. crassirostris on the prochlorophyte, (2) compare grazing rates on the HAB species with those on a palatable alternative (I. galbana), and (3) examine selective grazing when the HAB species is presented together with the palatable alternative at equal densities

  • Copepod mortality was not observed in monocultures of I. galbana

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Summary

Introduction

Harmful algal blooms (HABs) are increasing in estuaries around the world (Smayda, 2008; Berry et al, 2015; Phlips et al, 2015; Cao et al, 2017). In contrast to blooms dominated by dinoflagellates or diatoms (Phlips et al, 2015), the superbloom was co-dominated by cyanobacteria and a nanoplanktonic chlorophyte. Both taxa were documented at > 1 million cells ml−1 during the event (IRL Consortium, 2015). In addition to bottom-up stimulation by nutrients, the relatively small size and unusually high densities of the species in the superbloom led to questions about top-down control by herbivores like zooplankton

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