Modelling ecological change over half a century in a subtropical estuary: impacts of climate change, land-use, urbanization and freshwater extraction

Abstract

The Clarence River Estuary is the largest estuary in southeast Australia, with an extensive floodplain encompassing multiple river channels and a large coastal lagoon. It is the focus of major commercial and recreational fisheries and there is pressure to divert its freshwater inputs for agricultural and domestic uses. We used a spatial biogeochemical model to simulate the variability and evolution of this system on timescales from days to decades over the past half century. Like most tropical and subtropical estuaries, the Clarence River Estuary is strongly influenced by river discharge, sediment and nutrient loads. Given the high nutrient loads arriving from the upper catchment, plankton biomasses in the model were typically limited by flushing through the estuary channels. However, the longer residence times of the lagoons produced a profoundly different regime where higher zooplankton concentrations were supported by recycling of nutrients and detritus. Using alternative model scenarios, it was found that the ecology of the lagoon was sensitive to changes in land-use and urbanization within the local sub-catchment, but was largely insensitive to changes in upstream river discharge (i.e. rainfall or freshwater extraction). The opposite was true in the estuary channels, where changes in land-use or urbanisation in the populated lower catchment had little effect on the estuarine ecology, while even modest reductions in river discharges dramatically increased the biomass of the smaller phytoplankton and zooplankton groups, and favoured benthic algae over seagrass and macroalgae. The contrasting responses of these 2 estuarine environments suggests the need for distinct management approaches, with stringent controls on nutrient loads into coastal lagoons and protection of environmental flows into estuary channels.

Modeling ecosystem processes with variable freshwater inflow to the Caloosahatchee River Estuary, southwest Florida. I. Model development

“…Effective management of watershed and estuary resources requires the ability to link freshwater inflows to downstream ecological responses. Mathematical models accommodate this linkage by integrating many sources of information while providing a platform to conduct exploratory experiments (Sohma et al, 2008;Swaney et al, 2008;Buzzelli, 2011;Condie et al, 2012).…”

“…The quantification and evaluation of multiple stressors on estuarine ecology from days to decades requires an integrative framework linking watershed inputs, circulation, water quality, and biota (Sohma et al, 2008;Condie et al, 2012). Focused models can be used to explore potential ecological feedbacks and responses to variations in environmental drivers including altered inflow, temperature, light and nutrients, and grazing (Scavia and Liu, 2009;Lucas et al, 2009).…”

Benthic estuarine communities in Brazil: moving forward to long term studies to assess climate change impacts

“…For example, population dynamics and benthic annual production rates may vary significantly on annual and decadal scales due to changes in estuarine productivity, temperature, disturbance regimes, catastrophic events and interacting factors (KOTTA et al, 2009;DOLBETH et al, 2011). Although functional indices such as productivity regimes offer advantages in depicting ecosystem-wide responses, large variability in productivity may occur between estuaries due to site-specific differences such as pelagic productivity (CONDIE et al, 2012). Therefore, the multiple mechanisms that lead to changes in production and population dynamics over long-term periods suggest that estuarine ecosystems must be investigated on multiple scales.…”

“…However, it is still extremely difficult to predict the intensity and scale of these changes and the response of biological communities and changes in ecosystem functioning. Temperature and rainfall anomalies, as well as sea level rise, have been commonly reported across the globe and these effects may have substantial impacts on estuarine ecosystems over both the short-and long-term (ALONGI, 2008;DAY et al, 2008;CONDIE et al, 2012;TURRA et al, 2013;GARCÍA-RODRIGUEZ ET AL., 2014). For example, if regional and local rainfall anomalies and sea level changes alter the salt balance of an estuary, it might cause changes in species distribution and productivity (THURMAN et al, 2010;CONDIE et al, 2012).…”

“…Temperature and rainfall anomalies, as well as sea level rise, have been commonly reported across the globe and these effects may have substantial impacts on estuarine ecosystems over both the short-and long-term (ALONGI, 2008;DAY et al, 2008;CONDIE et al, 2012;TURRA et al, 2013;GARCÍA-RODRIGUEZ ET AL., 2014). For example, if regional and local rainfall anomalies and sea level changes alter the salt balance of an estuary, it might cause changes in species distribution and productivity (THURMAN et al, 2010;CONDIE et al, 2012). Higher temperatures could also affect the metabolism, growth and reproduction of estuarine biota, which combined with local eutrophication may lead to oxygen depletion and mass mortality of organisms (BISHOP et al, 2006).…”