Coupling between Marine Plankton and Freshwater Flow in the Plumes off a Small Estuary

Abstract

Freshwater discharge from rivers is a powerful forcing agent in coastal ecosystems. It not only generates strong ecological effects in estuaries, but also drives the dynamics of nearshore marine waters where prominent river plumes form biogeochemical hot spots in coastal seas worldwide. Large plumes from major rivers exert important controls on pelagic processes. The majority of estuaries are smaller, however, and the importance of the smaller plumes they generate is unknown. We measured the degree of coupling between freshwater flow and inshore zooplankton in such a plume from a subtropical estuary on the east coast of Australia. Flow regimes encompassed long periods of low freshwater input, punctuated by pulsed freshets that initiated the formation of buoyant, lower-salinity plumes in the nearshore marine zone. Plumes stimulated phytoplankton biomass in the receiving waters, and ultimately changes in zooplankton assemblages. Zooplankton responded strongly to river discharge: (1) in the absence of substantial freshwater flows and plumes, zooplankton was broadly similar in density and biomass across the estuarine-marine gradient; (2) freshets that generated significant plumes strongly modified hydrological conditions and lowered zooplankton in the estuarine and nearshore waters, and (3) after the initial freshet, zooplankton in the residual plume was at a higher density in nearshore than shelf waters. We demonstrate that coupling between riverine and coastal pelagic systems operates in small plumes, but that there is substantial temporal variance linked to fluctuations in freshwater delivery. IntroductionFreshwater inflow is one of the fundamental drivers of coastal ecosystems. The amount and temporal modes of freshwater discharge profoundly influence a wide range of biological processes at multiple levels of ecological organisation in estuaries, ranging from shifts * Corresponding author 642 T. A. SCHLACHER et al.

Use of local ecological knowledge in the management of algal blooms

“…Nutrients were the second most frequently cited 'cause' of the blooms in our study, and they fuel algal blooms of many species in a variety of settings (Valiela et al 1997;Cloern 2001;Howarth 2008;Schlacher et al 2008). Low levels of nitrogen can sustain high rates of growth in Hincksia sordida in laboratory experiments, and the alga is physiologically highly active in the surf zone where it can grow rapidly (Lovelock et al 2008).…”

“…Discharges from estuaries to nearshore marine waters in the form of prominent turbidity plumes can be a significant source of nutrient inputs from the land to the ocean on this coast and elsewhere (Gaston et al 2006). Marine plankton and benthos in receiving waters incorporate nitrogen and carbon delivered by these plumes (Schlacher et al 2008;Schlacher & Connolly 2009;Connolly et al 2010), and this may be a putative mechanism to stimulate algal blooms. Plumes from the Noosa estuary are, however, generally small and ephemeral and we have not observed any obvious concurrence between estuarine discharge events and algal blooms (T.A.…”

Potential effects of climate change on Australian estuaries and fish utilising estuaries: a review

“…Nutrients and light are the two key factors controlling phytoplankton growth (Eyre 2000). Phytoplankton biomass and productivity, timing and intensity of seasonal blooms and spatial and temporal composition and diversity are all also linked to freshwater inflow (Schlacher et al 2008). The abundance, distribution and assemblage structure of zooplankton and other organisms are in turn influenced by the characteristics of phytoplankton assemblages (Schlacher et al 2008;Brown et al 2010).…”

“…Phytoplankton biomass and productivity, timing and intensity of seasonal blooms and spatial and temporal composition and diversity are all also linked to freshwater inflow (Schlacher et al 2008). The abundance, distribution and assemblage structure of zooplankton and other organisms are in turn influenced by the characteristics of phytoplankton assemblages (Schlacher et al 2008;Brown et al 2010). The distribution and abundance of primary production in oceanic systems has already been linked to climate change, leading to changes in higher trophic levels (Richardson and Schoeman 2004;Beaugrand et al 2008).…”

“…Connolly et al 2009), and freshwater discharge from estuarine plumes may stimulate phytoplankton and subsequently zooplankton assemblages and fisheries offshore (e.g. Schlacher et al 2008).…”