Ecosystem change in a South African marine reserve (1960–2009): role of seagrass loss and anthropogenic disturbance

Abstract

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.

Spatial variation in abundance and diversity of the smaller surface and near‐surface eelgrass‐associated intertidal macrobenthos within a warm‐temperate estuarine bay in the Garden Route National Park, RSA

“…Insofar as the smaller members of the fauna at the upper levels of the Nanozostera beds are concerned, and except in the vicinity of the large pits excavated in the sediment, at least the Knysna system appears to be resilient to current bait-digging levels. It has also provedthus far -to have escaped loss of seagrass on the scale that has afflicted Langebaan over the last 50 years (Pillay et al, 2010a), the losses in the Outer Basin referred to in the Introduction having been almost completely offset by gains in the Middle Basin (CES, 2009), and thus its importance as a habitat for seagrass in southern Africa cannot be overestimated. …”

“…Species such as Hydrobia knysnaensis and 'Rissoa' capensis 1 were abundant in both Nanozostera and associated areas of bare sediment located within blocks of marsh but scarce or absent from fronting eelgrass beds along the lagoon shore. Besides being of considerable ecological interest in respect of the processes structuring assemblages of species, such spatial variation in the fauna of seagrass beds has very clear implications for strategies to conserve their organismal diversity and rare species (McNeill and Fairweather, 1993;Butler and Jernkoff, 1999;Green and Short, 2003;Larkum et al, 2006); something of particular importance at a time of worldwide seagrass decline (Duarte, 2002;Orth et al, 2006;Duarte et al, 2008;Hughes et al, 2009;Pillay et al, 2010a).…”

It’s All in the Genes: How Genotype Can Impact Upon Response to Contaminant Exposure and the Implications for Biomonitoring in Aquatic Systems

“…Most species of aquatic invertebrates feed by filtering or ingesting nutrients from the water or sediment, and thus can accumulate high concentrations of toxicants or contaminants that may be present within a habitat. It has been observed that population disruption or loss of bottom-level organisms and benthic invertebrate species can result in the disruption and even collapse of entire ecosystems (Owens & Dittman, 2003;Pillay et al, 2010;Wallace & Webster, 1996).…”

Influence of starfish grazing on lagoonal microalgal communities: non-competitive mechanisms for unimodal effects on diversity

“…Both studies also indicate that this species has been underrated in terms of its ability to structure benthic marine communities. The present study is also motivated by the fact that P. exigua occurs almost exclusively in and around seagrass (Zostera capensis) beds in Langebaan Lagoon and that the abundance of both species has been declining in the system (Pillay et al 2010b). This experiment is therefore important in understanding the consequences of further losses of this starfish for intertidal sandflats in Langebaan Lagoon.…”