Ecological risk assessment of metal pollutants in two agriculturally impacted estuaries.

A focused diagnosis of ecosystem health in two South African estuaries (Kromme and Gamtoos) was conducted. Four pollution indices were used, i.e., geoaccumulation (Igeo), ecological risk (RI), contamination factor (CF) and pollution load index (PLI), to assess toxicity levels of metal contaminants in relation to background values. The Igeo results (11.1 %) can be classified as contaminated, with Cd, the only element with high values in both estuaries. Likely sources (herbicides, pesticides) of Cd are used in the agricultural dominated catchments. There was a high concentration of Mn (13.4 ± 2.51 and 12.3 ± 1.13 µg·g-1) and Fe (1289 ± 243 and 1291 ± 130 µg·g-1) at site 4 for Gamtoos and Kromme estuary respectively compared to the other metal elements. Although results indicate low metal contamination, with increasing global anthropogenic pressure, continuous monitoring should be prioritised to assist in managing estuarine systems that support a wide range of socio-economic and ecosystem services.

Indirect effects shape macroalgal epifaunal communities.

We tested the response of algal epifauna to the direct effects of predation and the indirect consequences of habitat change due to grazing and nutrient supply through upwelling using an abundant intertidal rhodophyte, Gelidium pristoides. We ran a mid-shore field experiment at four sites (two upwelling sites interspersed with two non-upwelling sites) along 450 km of the south coast of South Africa. The experiment was started in June 2014 and ran until June 2015. Four treatments (predator exclusion, grazer exclusion, control, and procedural control) set out in a block design (n = 5) were monitored monthly for algal cover for the first 6 months and every 2 months for the last 6 months. Epifaunal abundance, species composition, algal cover, and algal architectural complexity (measured using fractal geometry) were assessed after 12 months. Predation had no significant effect on epifaunal abundances, while upwelling interacted with treatment. Grazing reduced the architectural complexity of algae, with increased fractal dimensions in the absence of grazers, and also reduced algal cover at all sites, though the latter effect was only significant for upwelling sites. Epifaunal community composition was not significantly affected by the presence of herbivores or predators but differed among sites independently of upwelling; sites were more similar to nearby sites than those farther away. In contrast, total epifaunal abundance was significantly affected by grazing, when normalized to algal cover. Grazing reduced the cover of algae; thus, epifaunal abundances were not affected by the direct top-down effects of predation but did respond to the indirect effects of grazing on habitat availability and quality. Our results indicate that epifaunal communities can be strongly influenced by the indirect consequences of biotic interactions.

Ectoparasites reduce scope for growth in a rocky-shore mussel (Perna perna) by raising maintenance costs.

Endolithic cyanobacteria are ubiquitous colonisers of organic and inorganic carbonate substrata that frequently attack the shells of mussels, eroding the shell to extract carbon, often with population infestation rates of >80%. This reduces host physiological condition and ultimately leads to shell collapse and mortality, compromising the services provided by these important ecosystem engineers. While the ecological implications of this and similar interactions have been examined, our understanding of the underlying mechanisms driving the physiological responses of infested hosts remains limited. Using field and laboratory experiments, we assessed the energetic costs of cyanobacterial infestation to the intertidal brown mussel (Perna perna). In the field we found that growth (measured as both increase in shell length and rate of biomineralization) and reproductive potential of clean mussels are greater than those of infested individuals. To explore the mechanisms behind these effects, we compared the energy allocation of parasite-free and infested mussels using the scope for growth (SFG) framework. This revealed a lower SFG in parasitized mussels attributed to an energetic imbalance caused by increased standard metabolic rates, without compensation through increased feeding or reduced excretion of ammonia. Separate laboratory assays showed no differences in calcium uptake rates, indicating that infested mussels do not compensate for shell erosion through increased mineralization. This suggests that the increased maintenance costs detected reflect repair of the organic component of the inner nacreous layer of the shell, an energetically more demanding process than mineralization. Thus, parasite-inflicted damage reduces SFG directly through the need for increased basal metabolic rate to drive shell repair without compensatory increases in energy intake. This study provides a first perspective of the physiological mechanisms underlying this parasite-host interaction, a critical step towards a comprehensive understanding of the ecological processes driving dynamics of this intertidal ecosystem engineer.