Search

Despite knowledge that seagrass meadows are threatened by multiple global change stressors, significant gaps exist in current knowledge. In particular, little is known about the interactive effects of warming and eutrophication on seagrasses globally, or about responses of African seagrasses to global change, despite these ecosystem engineers providing critical goods and services to local livelihoods. Here, we report on laboratory experiment assessing the main and joint effects of warming and nutrient enrichment on Cape eelgrass (Zostera capensis) from the West coast of South Africa, in which morphological attributes, photosynthetic efficiency and elemental content were assessed. Results indicate that shoot density, leaf length, aboveground biomass and effective quantum yield were negatively impacted by both warming and nutrient enrichment. Growth rate, leaf density and leaf width decreased with increasing nutrient levels but not temperature. In addition, epiphytic fouling on seagrass leaves were enhanced by both warming and nutrient enrichment but with warming eliciting a greater response. Collectively, our findings indicate a stronger effect of enrichment on Z. capensis performance relative to warming, suggesting that the upper levels of coastal eutrophication upon which our experiment was based is likely a stronger stressor than warming. Our findings also highlight limited interaction between warming and nutrient enrichment on Z. capensis performance, suggesting that effects of these stressors are likely to be propagated individually and not interactively. Our findings raise awareness of susceptibility of Z. capensis to eutrophication and the need to manage nutrient inputs into coastal ecosystems to preserve meadows of this seagrass and the critical ecosystem functions they provide. [ABSTRACT FROM AUTHOR]

Abstract: Heuweltjies are large circular earth mounds created by termites and are important features of the landscape in the Succulent Karoo biome in South Africa. This study demonstrates that heuweltjies and livestock grazing act together to shape vegetation patterns in the region. Using a multifactorial approach, we investigated the combined effects of heuweltjie-induced habitat conditions, grazing pressure and physical disturbance by livestock and native animals as well as the interactions among two distinct plant functional groups, on the composition and cover of vascular plant vegetation. Heuweltjie soils had significantly higher pH-values, finer texture, and higher concentrations of carbon, nitrogen, and phosphorus than those of the surroundings. Neither perennial nor ephemeral cover was different on- and off-mounds. However, species richness, cover of highly palatable plants and livestock grazing pressure were significantly higher between mounds. Grazing pressure generally resulted in a loss of perennial species and a shift to more ephemeral species, probably through the competitive release of annuals and geophytes from perennial plants. We propose that the apparent contradiction between superior nutrient conditions and inferior plant food quality on heuweltjies is caused by a combination of otherwise unfavourable soil properties and historic overgrazing of heuweltjies resulting in local degradation of the vegetation. [Copyright &y& Elsevier]

Summary: Knowledge of wetland systems from sub-humid and semi-arid regions remains poor, particularly with regards to surface water–groundwater interactions. As a result of variable inflow and high evapotranspiration rates, such systems are often associated with the development of groundwater salinity. By focusing on the riparian species, Acacia xanthophloea, this study investigates the interaction between vegetation, groundwater, and occurrence of salinity on the Mkuze River floodplain, a seasonally dry, freshwater wetland. The relationship between groundwater chemistry and water table elevation suggests that these deep-rooted trees act as evapotranspirational pumps, selectively removing water and causing the subsurface concentration of solutes. Extensive root systems that reach the water table, coupled with high transpiration rates, result in local groundwater reaching electrical conductivities in excess of 20mS/cm, approximately 15–20 times higher than those commonly found elsewhere on the floodplain. In this environment, these trees appear tolerant of salinities that would be toxic to most other plants. Plant tissue ion concentrations indicate that solute exclusion is the dominant means for avoidance of salt toxicity, with root turnover a possible regulatory mechanism. Data presented support our hypothesis that these trees exert feedback interactions on groundwater and sediment chemistry. Transpiration results not only in the development of saline groundwater, which is likely to influence vegetation distribution, but also initiates the precipitation of less soluble minerals, such as CaCO3 and SiO2, which have the potential to modify sediment pH, hydraulic conductivity, and landscape topography. Spatial variation in chemical processing is thus likely to play a role in creating and maintaining habitat diversity on the floodplain. Wetlands in semi-arid and sub-humid regions are often susceptible to shallow groundwater chemical transformations due to seasonal or episodic inflows and higher evapotranspiration demand. The documentation of solute concentration and retention in wetland systems from a variety of semi-arid and sub-humid areas suggests that evapotranspiration-driven processes may be more widespread than is currently understood. In environments where evapotranspiration plays an important role in the overall water budget of a wetland, similar vegetation–groundwater interactions and chemical processes are likely to occur. Recognition of broad differences between such systems and those of their better studied counterparts in tropical and temperate regions makes it necessary to develop a greater understanding of these processes. [ABSTRACT FROM AUTHOR]

Temperature and primary production (often linked to nutrient supply) are two of the few factors influencing species diversity and abundances across mesoscale gradients, while at smaller scales the habitat complexity offered by bioengineers is important. Previous studies have illustrated effects of upwelling and biogeography on intertidal bioengineers, but it is not known if these proëesses influence assemblages associated with those bioengineers in a similar way. We examined the habitat structure offered by two species of mussels and their associated fauna in five regions across 3000 km and three biogeographic provinces of the South African coast, replicating upwelling and. non-upwelling areas within each region. Upwelling and region influenced the structure of mussel beds (the density and size of mussels). In contrast, upwelling did not influence mesoscale differences in composition, abundance, .and numbers of species of crustaceans, mollusks, or polychaetes in mussel beds. Regardless of trophic level or mode of reproduction, mussel bed fauna were influenced only by region. Regional differences were strongly influenced by biogeography. The associated fauna was, however, also strongly correlated with the structure of the habitats created by mussels. Our results support the importance of upwilling to a critical ecosystem engineer, but show that these effects do not extend directly to the assemblages of associated fauna, which are more influenced by regional-scale effects and biogeography. We suggest that mesoscale patterns in the associated fauna of this bioengineered habitat are driven by the direct effects of biogeography, combined with the influence of biogeography and upwelling on mussel bed structure. [ABSTRACT FROM AUTHOR]