Abstract
Drift macroalgae plays key roles in the ecology of many coastal systems, including the Indian River Lagoon. In the lagoon, changes in the biomass of drift macroalgae may have interacted with an unprecedented bloom of phytoplankton in 2011. Patterns in the biomass of drift macroalgae were identified using new and original analyses of data from several sampling programs collected between 1997 and 2019. All available data show a relatively low biomass of drift macroalgae in 2010–2012, and surveys of fixed transects and seining as part of a fisheries independent monitoring program also recorded low biomass in 2016. Low light availability and potentially stressful temperatures appeared to be the main influences as indicated by the results of incubations in tanks to determine environmental tolerances and data on ambient conditions. Decreased biomass of drift macroalgae had implications for cycling of nutrients because carbon, nitrogen, and phosphorus not stored in the tissues of drift macroalgae became available for uptake by other primary producers, including phytoplankton. The estimated 14–18% increases in concentrations of these elements in the IRL could have promoted longer and more intense phytoplankton blooms, which would have reduced light availability and increased stress on algae and seagrasses. An improved understanding of such feedback and the ecological roles played by drift macroalgae will support more effective management of nutrient loads and the system by accounting for cycling of nutrients among primary producers.
Highlights
Interactions among three key primary producers, phytoplankton, macroalgae, and rooted macrophytes, represent influential processes in many estuaries (Duarte, 1995; Kinney and Roman, 1998; Viaroli et al, 2008)
Biomass of drift macroalgae (DMA) varied significantly in space and through time, widespread decreases in biomass were related to thresholds for environmental tolerances as determined by incubations in the laboratory and field data, and less DMA translated into less carbon, nitrogen and phosphorus being stored in DMA
High biomass was recorded in different seasons, years, and reaches by the different methods, but low biomass was recorded by multiple sampling methods during 2010–2012 and in 2016, especially in reaches 1 through 6 where DMA was more common and abundant
Summary
Interactions among three key primary producers, phytoplankton, macroalgae, and rooted macrophytes, represent influential processes in many estuaries (Duarte, 1995; Kinney and Roman, 1998; Viaroli et al, 2008). Shifts from dominance by benthic primary producers to dominance by phytoplankton have been observed in multiple systems with negative impacts on seagrass assemblages and their associated fauna (Jensen and Gibson, 1986; Duarte, 1995; Burkholder et al, 2007; Duarte et al, 2010). Such a shift may have occurred in the IRL because an unprecedented sequence of intense and longlasting blooms of phytoplankton has afflicted the system since 2011 (Phlips et al, 2010, 2011, 2015)
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