Variation in rocky shore assemblages and abundances of key taxa along gradients of stormwater input

Highlights

• Taxon richness and algal cover on rocky shores increased between 10 and 60 m from three stormwater outfalls.

• In most cases there was a step up in diversity after 20 m.

• Assemblage structure at or near stormwater sites differed from that at sites 100 m away.

Abstract

Stormwater brings freshwater and terrestrially derived contaminants into coastal systems and is predicted to increase with climate change. This study aimed to characterise variation in rocky shore assemblages in relation to stormwater pollution. Intertidal assemblages were sampled in similar habitats at a range of distances (0 m, 10 m, 20 m, 60 m, and 100 m) from stormwater outfalls on three rocky shores north of Dublin. In general, taxon richness and algal cover increased after 20 m from a stormwater outfall. Limpet population structure and condition index showed no consistent patterns among shores. Assemblage structure at or near stormwater sites differed from that at sites 100 m away. These findings, ideally supplemented by experimental research, may be used to inform stormwater management and remediation approaches.

Introduction

Society relies on marine ecosystems for a range of ecosystem services, such as provision of food, nutrient recycling and climate regulation (Millenium Ecosystem Assessment, 2003, TEEB, 2010, Liquete et al., 2013). Multiple anthropogenic stressors can alter the functioning of coastal ecosystems, reducing biodiversity and the provision of services to society (Johnston and Roberts, 2009, Bulling et al., 2010). Increased urbanization has led to an increase in a wide range of pollutants deposited on catchment surfaces which are transported with stormwater during wet weather periods and ultimately enter aquatic systems (Aryal et al. 2010). Significant changes are projected to occur in climate over this century, with an expected increase of 10–25% for Ireland's winter precipitation (McGrath et al. 2005). The generation of stormwater and its associated pollutants is determined by the level of precipitation that occurs (Patz et al. 2008), and is also affected by rainfall-related flooding (Dierkes et al. 2002).

Stormwater is an important uncontrolled and unregulated source of pollution. The creation of stormwater pollutants in urban environments is complex and the pollutants arise from a large number of urban activities (Duncan, 1999). It can degrade water quality by introducing a combination of contaminants such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), heavy metals, freshwater, pesticides, sediment, nutrients, bacteria and sewerage (Makepeace et al. 1995). Stormwater can alter the quality, turbidity, salinity, temperature and pH of the water it enters (Corcoran et al. 2010) causing significant effects on marine biota (Pratt et al. 1981). The impact of stormwater pollutants on organisms depends on a number of factors such as total load of pollutants, their nature and concentration. Anthropogenic stressors such as this have the potential to alter the diversity and structure of natural assemblages.

To date the ecological effects of stormwater run-off in marine ecosystems remains uncertain. Many stormwater studies have focused on the toxicity of stormwater on individual organisms (Schiff et al., 2003, Greenstein et al., 2004, Grapentine et al., 2008). Multiple interacting species in an assemblage serve as a better representation of an ecosystem as the contaminants may act as a stressor on any of the complex dynamics that affect assemblage integrity (Maher and Norris, 1990). A number of studies have explored the relationship between stormwater pollution and marine assemblage structure (Willemsen et al., 1990, Morrissey et al., 2003, Schiff and Bay, 2003, Ghedini et al., 2011) yet only weak biological effects have been observed. Many of these studies have looked at short term effects (i.e. single rainfall events). Further study is needed of the longer term changes associated with chronic inputs of stormwater and its cumulative effects. Benthic communities provide a useful model for marine pollution assessment as they reflect not only conditions at the time of sampling but also conditions to which the community has previously been exposed (Reish, 1986). Some taxa, such as perennial algae and some grazing gastropods are particularly influential and may be sensitive to pollution (Borowitkza, 1972, Philips, 1977, Evans-White and Lamberti, 2009). Furthermore, the ratio of ephemeral to perennial algae can be used as an indicator of ecological status (Cusack et al. 2008) and it can be informative to characterise the effects on these groups of species. Changes in populations of these taxa would have potentially important indirect effects for the wider community and ecosystem processes (e.g. Menge, 1995, O'Connor and Crowe, 2005, Crowe et al., 2013).

The aim of this study was to relate the distance from a stormwater outfall with the structure and taxon richness of rocky shore assemblages as well as with the abundance of selected taxa and with size structure and condition of limpets.