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Abstract
Subtropical systems experience occasional severe floods, dramatically altering the phytoplankton community structure, in response to changes in salinity, nutrients, and light. This study examined the effects of a 1:100 year summer flood on the phytoplankton community in an Australian subtropical bay – Moreton Bay – over 48 weeks, from January to December 2011. Immediately after maximum flood levels were reached on the rivers flowing into the bay, the lowest salinity, and highest turbidity values, in more than a decade, were measured in the Bay and the areal extent of the flood-related parameters was also far greater than previous flood events. Changes in these parameters together with changes in Colored Dissolved Organic Matter (CDOM) and sediment concentrations significantly reduced the light availability within the water column. Despite the reduced light availability, the phytoplankton community responded rapidly (1–2 weeks) to the nutrients from flood inputs, as measured using pigment concentrations and cell counts and observed in ocean color satellite imagery. Initially, the phytoplankton community was totally dominated by micro-phytoplankton, particularly diatoms; however, in the subsequent weeks (up to 48-weeks post flood) the community changed to one of nano- and pico-plankton in all areas of the Bay not usually affected by river flow. This trend is consistent with many other studies that show the ability of micro-phytoplankton to respond rapidly to increased nutrient availability, stimulating their growth rates. The results of this study suggest that one-off extreme floods have immediate, but short-lived effects, on phytoplankton species composition and biomass as a result of the interacting and dynamic effects of changes in nutrient and light availability.
Highlights
Coastal embayments are complex water systems influenced by both the marine waters of off-shore seas and the terrestrial runoff carried by freshwater flow through rivers and estuaries
The MODIS image from 22 January 2011 showed that the freshwater plume was still affecting surface waters of the bay, but the very high aCDOM(440) values had receded toward the coast (Figure 2A)
In some of these studies only concentrations of Chl-a were measured as an indicator of total phytoplankton biomass (Moss, 1998; Uwins et al, 1998), whilst other studies utilized microscopic analyses to measure phytoplankton abundance (Heil et al, 1998a,b)
Summary
Coastal embayments are complex water systems influenced by both the marine waters of off-shore seas and the terrestrial runoff carried by freshwater flow through rivers and estuaries. The location of the bay will affect the phytoplankton community dynamics with temperate systems receiving highest river flow during winter, showing greater fluctuation in temperature and typically having maximum biomass in spring (Eyre, 2000; Ansotegui et al, 2003; Cook et al, 2010; Schaeffer et al, 2012). When the system is disturbed by an extremely large flow or flood event, different populations are disturbed and the phytoplankton community composition can become more homogeneous throughout the bay (Angeler et al, 2000; Dagg et al, 2004). Recovery to the phytoplankton communities that existed pre-flood event will often take much longer than the flood takes to subside (Davies and Eyre, 1998; Moss, 1998; Eyre, 2000; Weyhenmeyer et al, 2004; Muylaert and Vyverman, 2006; Guizien et al, 2007)
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