State of the Bay Report 2011-Final.pdf - Anchor Environmental

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State of Saldanha Bay & Langebaan Lagoon 2011 Aquatic macrophytes 6 AQUATIC MACROPHYTES IN LANGEBAAN LAGOON Three distinct intertidal habitats exist within Langebaan Lagoon: seagrass beds, such as those of the eelgrass Zostera capensis; salt marsh dominated by cordgrass Spartina maritime and Sarcocornia perennis; and unvegetated sandflats dominated by the sand prawn, Callianassa krausii and the mudprawn Upogebia capensis (Siebert and Branch 2005a,b). Sand and mud pawns are considered ecosystem engineers as their feeding and burrowing activities modify the local environmental conditions, which in turn modify the composition of the faunal communities (Rhoads and Young 1970, Woodin 1976, Wynberg and Branch 1991). Seagrass beds and salt marshes perform an opposite and antagonistic engineering role to that of the sand and mud prawns as the root-rhizome networks of the seagrass and saltmarsh plants stabilize the sediments (Siebert and Branch 2005a). In addition, the three dimensional leaf canopies of the seagrass and saltmarsh plants reduce the local current velocities thereby trapping nutrients and increasing sediment accretion (Kikuchi and Peres 1977; Whitfield 1989, Hemmingra and Duarte 2000). The importance of seagrass and saltmarsh beds as ecosystem engineers has been widely recognized. The increased food abundance, sediment stability, protection from predation and habitat complexity offered by seagrass and saltmarsh beds provide nursery areas for many species of fish and invertebrates and support, in many cases a, higher species richness, diversity, abundance and biomass of invertebrate fauna compared to unvegetated areas (Kikuchi and Peres 1977, Whitfield 1989, Hemmingra and Duarte 2000, Heck et al. 2003, Orth et al. 2006, Siebert and Branch 2007). Seagrass and saltmarsh beds are also important for waterbirds some of which feed directly on the shoots and rhizomes, forage amongst the leaves or use them as roosting areas at high tide (Baldwin & Lovvorn 1994, Ganter 2000, Orth et al. 2006). Seagrass Saltmarsh Figure 6.1. Seagrass (black) and saltmarsh (green) near Bottelary in Langebaan Lagoon. Source: Google Earth. 124 ANCHOR e n v i r o n m en t a l
State of Saldanha Bay & Langebaan Lagoon 2011 Aquatic macrophytes 6.1 Long term changes in seagrass in Langebaan Lagoon Seagrass beds are particularly sensitive to disturbance and are declining around the world at rates comparable to the loss of tropical rainforests, placing them amongst the most threatened ecosystems on the planet (Waycott et al. 2009). The loss of seagrass beds is attributed primarily to anthropogenic impacts such as coastal eutrophication, alterations to food webs caused by the overexploitation of predatory fish, and modified sediment dynamics associated with coastal and harbour development (Waycott et al. 2009). The loss of seagrass meadows has been shown to have profound implications for the biodiversity associated with them, including loss of invertebrate diversity, fish populations, that use the sheltered habitat as nurseries, and waterbirds, that use the seagrass meadows as foraging grounds during their non-breeding period (Hughes et al. 2002). Long-term changes in seagrass beds in Langebaan Lagoon have been investigated by Angel et al. 2006 and Pillay et al. (2010). Angel et al. (2006) focused on long term trends at Klein Oesterwal and Bottelary, and was able to show that the width of the Z. capensis bed changed substantially between 1972 and 2004, with three major declines evident in this period (Figure 6.2). The first occurred in the late 1970s, and was followed by a slow recovery in the early 1980’s, the second occurred between 1988 and 1993 and the third between 2002 and 2004 (Angel et al. 2006). Mirroring this decline were the striking fluctuations of the small endemic limpet Siphonaria compressa, which lives on the leaves of Z. capensis and is completely dependent on the seagrass for its survival. The densities of S. compressa collapsed twice in this period to the point of local extinction, corresponding with periods of reduced seagrass abundance (Figure 6.2). At Bottelary, the width of the seagrass bed and densities of S. compressa followed the same pattern as at Klein Oesterwal, with a dramatic collapse of the population between 2002 and 2004, followed by a rapid recovery in 2005 (Angel et al. 2006). The first decline in seagrass cover coincided with blasting and dredging operations in the adjacent Saldanha Bay, but there is no obvious explanation for the second decline (Angel et al. 2006). Pillay et al. (2010) documents changes in seagrass Zostera capensis abundance at four sites in the Lagoon – Klein Oesterwal, Oesterwal, Bottelary and the Centre banks using a series of aerial photographs covering the period 1960 to 2007. During this time the total loss of Z. capensis amounted to 38% or a total of 0.22 km 2 across all sites. The declines were most dramatic at Klein Oesterwal where close to 99% of the seagrass beds were lost during this period, but were equally concerning at Oesterwal (82% loss), Bottelary (45% loss) and Centre Bank (18% loss) (Pillay et al. 2010). Corresponding changes were also observed in densities of benthic macrofauna at these sites, with species that were commonly associated with Zostera beds such as the starfish Parvulastra exigua and the limpets Siphoneria compressa and Fisurella mutabilis and general surface dwellers such as the gastropods Assiminea globules, Littorina saxatilis, and Hydrobia sp. declining in abundance, while those species that burrowed predominantly in unvegetated sand, such as amphopods Urothoe grimaldi and the polychaetes Scoloplos johnstonei and Orbinia angrapequensis increased in density. Pillay et al. (2010) was also able to show that the abundance of at least one species of wading bird Terek sandpiper which feeds exclusively in Zostera beds was linked to changes in the size of these beds, with population crashes in this species coinciding with periods of lowest seagrass abundance at Klein Oesterwal. By contrast, they were able to show that populations of wader species that do not feed in seagrass beds were more stable over time. While the precise reasons for the loss of Z. capensis beds remain speculative, the impact of human disturbance cannot be discounted, particularly at Klein Oesterwal where bait collection is common (Pillay et al. 2006). By 2007 the intertidal habitat at Klein Oesterwal had been transformed from a seagrass bed community to an unvegetated sand flat which was colonized by the burrowing sandprawn Callinassa kraussi and other sandflat species that cannot live in the stabilized sediments promoted by the seagrass (Pillay et al. 2010). The burrowing sandprawn turns over massive quantities of sediment and once established effectively prevents the re-colonization of seagrass and 125 ANCHOR e n v i r o n m en t a l
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