Abstract
In 2011, the Indian River Lagoon, a biodiverse estuary in eastern Florida (USA), experienced an intense microalgal bloom with disastrous ecological consequences. The bloom included a mix of microalgae with unresolved taxonomy and lasted for 7 months with a maximum concentration of 130 μg chlorophyll a L−1. In 2012, brown tide Aureoumbra lagunensis also bloomed in portions of this estuary, with reoccurrences in 2016 and 2018. To identify and understand the role of grazer pressure (top-down control) on bloom formation, we coupled DNA sequencing with bivalve feeding assays using three microalgae isolated from the 2011 bloom and maintained in culture. Feeding experiments were conducted on widely distributed bivalve species in the lagoon, including eastern oysters (Crassostrea virginica), hooked mussels (Ischadium recurvum), charru mussels (Mytella charruana), green mussels (Perna viridis), Atlantic rangia (Rangia cuneata), and hard clams (Mercenaria mercenaria), which were exposed to 3 × 104 cells mL−1 of five species of microalgae consisting of A. lagunensis and the three species clarified herein, the picocyanobacteria Crocosphaera sp. and ‘Synechococcus’ sp., and the picochlorophyte Picochlorum sp., as well as Nannochloropsis oculata used as a control. To ensure clearance rates were indicative of consumption and assimilation, the microalgae were isotopically (15N) labeled prior to feeding experiments. Clearance rates differed among bivalve and microalgal species, but enriched 15N values in bivalve tissue suggest that algal bloom species were assimilated by the bivalves. These results expand our understanding of the important ecosystem services that healthy, biodiverse filter feeder communities provide.
Highlights
Marine coastal ecosystems are under increased threat from eutrophication that is fueled by anthropogenically derived inputs like sewage, animal waste, and fertilizer (Halpern et al 2007)
We focused on Aureoumbra lagunensis and three other species that had been isolated at the onset of the Indian River Lagoon (IRL) 2011 bloom but had not been previously identified molecularly to resolve their taxonomy
Bivalves were maintained at 20 °C and the salinities in which they were collected (R. cuneata 5 ppt, M. mercenaria, I. recurvum, and C. virginica 20 ppt; M. charruana and P. viridis 30 ppt), except for I. recurvum individuals, which were collected at 5 ppt but slowly acclimated to 20 ppt over the period of 2 weeks, as this is the salinity in which they were more likely to encounter the microalgal species
Summary
Marine coastal ecosystems are under increased threat from eutrophication that is fueled by anthropogenically derived inputs like sewage, animal waste, and fertilizer (Halpern et al 2007). These excess nutrients can cause harmful algal blooms that have detrimental impacts on diverse ecosystems but have had an especially strong effect on estuaries In 2012, “brown tide” blooms dominated by Aureoumbra lagunensis (Stockwell et al, 1997), were first observed in the Mosquito Lagoon, Banana River, and northern Indian River (Phlips and Badylak 2013; Phlips et al 2015), reoccurred in parts of the region in 2013, 2015, 2016, and 2018 (Phlips et al 2019). Filter-feeding organisms, such as bivalves, may play a major role in preventing or remediating potential negative effects of microalgal proliferations
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