Plankton bioindicators of environmental conditions in coastal lagoons

Highlights

• We identify two distinct plankton communities in the Coorong.

• Salinity and nutrient are major factors shaping the communities.

• We identify two sets of positive and negative indicator species.

• We provide a tool for managing ecosystem health states of coastal lagoons.

Abstract

Coastal lagoons are characterised by strong spatial gradient of environmental parameters, especially hypersalinity, and are prone to anthropogenic disturbance. The Coorong (South Australia) is an inverse estuarine coastal lagoon separated from the sea by sand dunes. It is exposed to extreme water quality changes that affect its aquatic communities. Here, we used plankton as indicators of extreme environmental fluctuations to monitor and manage the environmental health of such complex systems. We defined the relationship of different plankton communities with water quality fluctuations and determined plankton species suitable for monitoring the ecosystem health. Two distinct communities of phytoplankton and zooplankton were identified, with salinity and nutrients being the principal factors impacting species distribution. Thus, two sets of indicator species were selected based on the different communities observed. Polychaete and gastropod larvae were positive indicators, showing salinity range restriction of brackish to marine. The distribution Acartia cf. fancetti represented healthy hypersaline conditions (salinity 40–60), while Cyclophora sp. and Scrippsiella sp. were negative indicators, correlating with extreme salinity and ammonia levels. The implementation of planktonic organisms as environmental indicators provided a constructive tool for the management of ecosystem health of the Coorong and will be applicable to similar coastal lagoons.

Introduction

A number of comprehensive and multidisciplinary studies (Lasserre and Marzollo, 2000, Roselli et al., 2013, Giménez et al., 2014, Coelho et al., 2015) have shown that the ecology of coastal lagoons is highly variable, depending on the prevalent physical and chemical environment. In particular, tidal to wind-driven water movements and strong spatial gradients of environmental parameters can be observed (Lamptey and Armah, 2008, Leterme et al., 2015). Coastal lagoons are prone to anthropogenic interference and disturbance by agricultural drainage, discharge of sewage or change in hydrology, thus modifying their structure and function (Brehmer et al., 2013, Dhib et al., 2013, Leterme et al., 2015). Such modifications, coupled with climatic variations, affect the overall ecosystem health and environmental gradients of specific coastal lagoons (Lester and Fairweather, 2009; Paul et al., 2016), therefore causing stress and changes in communities of resident species (Paul et al., 2016). The active interaction of climatic and anthropogenic stressors presents a serious challenge in terms of managing and predicting the water quality and ecosystem health of estuaries and coastal lagoons.

Plankton communities are often used as bioindicators to monitor ecological change in aquatic systems (Paul et al., 2016, Lemley et al., 2016) and are considered good natural bioindicators due to their rapid response to fluctuating environmental conditions (Livingston et al., 2002, Albaina et al., 2009, Amengual-Morro et al., 2012). In particular, phytoplankton populations reflect climate variability and dramatic changes that occur in aquatic ecosystems (Edwards and Richardson, 2004, Richardson and Schoeman, 2004, Leterme et al., 2005). Similarly, zooplankton communities are considered ideal bioindicators for estuarine conditions as they have the potential to remain in the water body of appropriate water quality (Wilson, 1994, Albaina et al., 2009). Assessing plankton dynamics is, therefore, of particular importance when considering the impact of environmental variability on ecological changes of coastal lagoons, that are amongst the most productive and dynamic habitats on earth (Alongi, 1998, Gönenç and Wolflin, 2005). However, fundamental information on how climatic variability affects population dynamics, abundance and taxonomy in those environments is still relatively limited (Anthony et al., 2009, Hall et al., 2013).

The Coorong lagoon (South Australia) is one of Australia's most important wetland areas and a RAMSAR listed wetland. It is known to have a high biological diversity, supporting a wide variety of local and migrating shore birds, fish populations, planktonic organisms, benthic macroinvertebrate populations and flora (Zampatti et al., 2010, Paton and Bailey, 2011, 2012; Shiel and Tan, 2013, Dittmann et al., 2015, Leterme et al., 2015). The Coorong is a long, narrow coastal lagoon at the mouth of Australia's largest river system, the Murray-Darling Basin (MDB). This shallow water system is connected to the sea at the mouth of the River Murray (Murray Mouth), which is subject to infilling and scouring on a seasonal basis. Unlike most estuaries, freshwater input from the MDB occurs close to the estuary mouth, and is primarily drawn along through the Coorong by salinity-driven gradients in response to evaporative water loss (Webster, 2010). The Coorong lagoon is a complex system as it also exhibits inverse estuarine characteristics, whereby evaporation exceeds the freshwater input (Pritchard, 1952; Lester and Fairweather, 2009). Moreover, salinity is higher at the head of the system than at the mouth (Leterme et al., 2015; Lester and Fairweather, 2009). The salinity levels of the Coorong have increased dramatically between 2004 and 2010 (>90) because of the lack of water inflow due to the increased retainment of water further up the Murray river (Nayar and Loo, 2009), but also due to the drought (2004–2010; Zampatti et al., 2010). Since 2010, however, there has been a substantial increase in freshwater release into the Coorong and the connectivity to the sea has improved. This resulted in a change of fauna and flora communities as clearly shown by Paton and Bailey (2014), Dittmann et al., 2015, Leterme et al., 2015.

Few studies have looked at the spatial distribution of planktonic organisms in hypersaline inverse estuaries such as the Coorong (Silva et al., 2009) and, therefore, relatively less is known compared to that in positive estuaries. Moreover, few studies have explored the use of planktonic organisms as ecosystem health and environmental variability indicator in such systems. Understanding and modelling the dynamics of an aquatic system without knowledge of its base of productivity is often not possible (Jaanus et al., 2009). In this study, we identify potential planktonic indicator species suitable for monitoring and managing environmental variability and health of ecosystems such as inverse estuaries. To achieve this, we study the spatial and temporal changes in plankton communities along the Coorong (phytoplankton and zooplankton) and look at their relation to environmental variability, also providing the first account of the zooplankton populations of the hypersaline part of the system. We hypothesise a significant decrease in the number of plankton species and the presence of a different plankton community in the hypersaline region. We identify the main water quality parameters that influence the plankton dynamics of the system and then categorise suitable species to serve as indicators of water quality variability and ecosystem health following the criteria given by Hilty and Merenlender, 2000, Carignan and Villard, 2002 and Siddig et al. (2016).