Seagrass ecosystems are disappearing throughout the world, raising several concerns for ecosystem functioning and biodiversity. In the present study we document changes in intertidal sandflat ecosystems over the last 5 decades in Langebaan Lagoon, a marine reserve on the west coast of South Africa, following large-scale losses of seagrass Zostera capensis. Aerial photography and GIS revealed a 38% loss of seagrass at sites in Langebaan Lagoon between 1960 and 2007. Longterm monitoring of one particular site, Klein Oesterwal, indicated a major shift in invertebrate communities between 1983, when Z. capensis was abundant, and 2009, when Z. capensis had virtually disappeared. Abundance and species richness of invertebrates declined significantly over this period. Seagrass-associated species such as the limpets Siphonaria compressa and Fissurella mutabilis, and the starfish Parvulastra exigua, declined almost to extinction locally. Generalists such as the gastropod Assiminea globulus also declined in abundance. However, sandflat species, particularly burrowers, increased in abundance and vertical range. Wading birds also appeared to be affected by changes in seagrass cover. The terek sandpiper, which depends on Z. capensis for feeding, showed 3 local population crashes, each corresponding to periods of seagrass collapse. Abundance of wading birds that were not dependant on this seagrass was more stable. The loss of seagrasses in the system, together with anthropogenic disturbance such as bait harvesting and trampling, may have had severe impacts on invertebrates, the most significant being the virtual extinction locally of the critically endangered limpet S. compressa. Cascading effects on wading birds, possibly through changes in availability of invertebrate prey, were also evident.

The alteration of sediments by bioturbating organisms plays a major role in aquatic ecosystems, from both ecological and evolutionary perspectives. Several studies have highlighted the ability of bioturbators to alter sedimentary biogeochemical processes and community structure, but the potential influence of bioturbators on pelagic species is unexplored in marine ecology. In the present study, we investigated the direct effects of bioturbation by a southern African burrowing sandprawn, Callichirus kraussi, on primary producers (benthic microalgae) and the indirect effects on the growth of a fish (grey mullet Liza richardsonii) that consumes microalgae. A mesocosm experiment was undertaken in which similar-sized L. richardsonii at 2 densities were exposed to 3 increasing densities of C. kraussi. After 3 wk of exposure to the effects of C. kraussi, the fish were weighed and their lengths measured to calculate their physical condition and growth rates. At the termination of the experiment, the microalgal biomass and sediment turnover were measured in each mesocosm. Higher C. kraussi densities resulted in an increase in sediment turnover and caused reductions in microalgal biomass, which in turn led to a reduction of the biomass and lengths of L. richardsonii. Increasing densities of C. kraussi evidently enhance sediment turnover from burrows to the sediment surface, leading to the burial of microalgae and indirect reductions in food availability to L. richardsonii. This reduction in turn leads to metabolic losses and reductions in the growth of this fish species. These results indicate that benthic bioturbators can have strong effects on aquatic ecosystems, especially by modulating energy flow to nektonic species. KEY WORDS: Bioturbation · Food webs · Marine benthos · Sediment turnover · Benthic-pelagic coupling Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 453: 151-158, 2012, Pillay & Branch 2011. Bioturbating organisms feature prominently in driving changes at both evolutionary and ecological scales. At the evolutionary level, burrowing species and bioturbation have been implicated in the Cambrian-substrate revolution, in which burrowers are thought to have contributed to the transition between pre-and postCambrian animal life in marine sediments (Meysman et al. 2006). Burrowing species, through their bioturbative activities, can also indirectly drive microevolutionary change that ultimately influences the evolution of novel morphology, behaviour and social interactions (Pillay 2010).At the ecological level, bioturbation influences a number of processes and properties, including nutrient fluxes, sediment granulometry and pore-water characteristics (Suchanek 1983, Lohrer et al. 2004, Meysman et al. 2006). These modifications translate into significant effects on the associated softsediment communities. Bioturbators have been shown to influence the community structure, abundance and distribution of the major benthic biotic components (Suchanek 1983, Flach...