State of the Bay Report 2011-Final.pdf - Anchor Environmental
State of the Bay Report 2011-Final.pdf - Anchor Environmental
State of the Bay Report 2011-Final.pdf - Anchor Environmental
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<strong>State</strong> <strong>of</strong> Saldanha <strong>Bay</strong> & Langebaan Lagoon <strong>2011</strong><br />
Aquatic macrophytes<br />
6.1 Long term changes in seagrass in Langebaan Lagoon<br />
Seagrass beds are particularly sensitive to disturbance and are declining around <strong>the</strong> world at rates<br />
comparable to <strong>the</strong> loss <strong>of</strong> tropical rainforests, placing <strong>the</strong>m amongst <strong>the</strong> most threatened<br />
ecosystems on <strong>the</strong> planet (Waycott et al. 2009). The loss <strong>of</strong> seagrass beds is attributed primarily to<br />
anthropogenic impacts such as coastal eutrophication, alterations to food webs caused by <strong>the</strong><br />
overexploitation <strong>of</strong> predatory fish, and modified sediment dynamics associated with coastal and<br />
harbour development (Waycott et al. 2009). The loss <strong>of</strong> seagrass meadows has been shown to have<br />
pr<strong>of</strong>ound implications for <strong>the</strong> biodiversity associated with <strong>the</strong>m, including loss <strong>of</strong> invertebrate<br />
diversity, fish populations, that use <strong>the</strong> sheltered habitat as nurseries, and waterbirds, that use <strong>the</strong><br />
seagrass meadows as foraging grounds during <strong>the</strong>ir non-breeding period (Hughes et al. 2002).<br />
Long-term changes in seagrass beds in Langebaan Lagoon have been investigated by Angel<br />
et al. 2006 and Pillay et al. (2010). Angel et al. (2006) focused on long term trends at Klein<br />
Oesterwal and Bottelary, and was able to show that <strong>the</strong> width <strong>of</strong> <strong>the</strong> Z. capensis bed changed<br />
substantially between 1972 and 2004, with three major declines evident in this period (Figure 6.2).<br />
The first occurred in <strong>the</strong> late 1970s, and was followed by a slow recovery in <strong>the</strong> early 1980’s, <strong>the</strong><br />
second occurred between 1988 and 1993 and <strong>the</strong> third between 2002 and 2004 (Angel et al. 2006).<br />
Mirroring this decline were <strong>the</strong> striking fluctuations <strong>of</strong> <strong>the</strong> small endemic limpet Siphonaria<br />
compressa, which lives on <strong>the</strong> leaves <strong>of</strong> Z. capensis and is completely dependent on <strong>the</strong> seagrass for<br />
its survival. The densities <strong>of</strong> S. compressa collapsed twice in this period to <strong>the</strong> point <strong>of</strong> local<br />
extinction, corresponding with periods <strong>of</strong> reduced seagrass abundance (Figure 6.2). At Bottelary, <strong>the</strong><br />
width <strong>of</strong> <strong>the</strong> seagrass bed and densities <strong>of</strong> S. compressa followed <strong>the</strong> same pattern as at Klein<br />
Oesterwal, with a dramatic collapse <strong>of</strong> <strong>the</strong> population between 2002 and 2004, followed by a rapid<br />
recovery in 2005 (Angel et al. 2006). The first decline in seagrass cover coincided with blasting and<br />
dredging operations in <strong>the</strong> adjacent Saldanha <strong>Bay</strong>, but <strong>the</strong>re is no obvious explanation for <strong>the</strong><br />
second decline (Angel et al. 2006).<br />
Pillay et al. (2010) documents changes in seagrass Zostera capensis abundance at four sites<br />
in <strong>the</strong> Lagoon – Klein Oesterwal, Oesterwal, Bottelary and <strong>the</strong> Centre banks using a series <strong>of</strong> aerial<br />
photographs covering <strong>the</strong> period 1960 to 2007. During this time <strong>the</strong> total loss <strong>of</strong> Z. capensis<br />
amounted to 38% or a total <strong>of</strong> 0.22 km 2 across all sites. The declines were most dramatic at Klein<br />
Oesterwal where close to 99% <strong>of</strong> <strong>the</strong> seagrass beds were lost during this period, but were equally<br />
concerning at Oesterwal (82% loss), Bottelary (45% loss) and Centre Bank (18% loss) (Pillay et al.<br />
2010). Corresponding changes were also observed in densities <strong>of</strong> benthic macr<strong>of</strong>auna at <strong>the</strong>se sites,<br />
with species that were commonly associated with Zostera beds such as <strong>the</strong> starfish Parvulastra<br />
exigua and <strong>the</strong> limpets Siphoneria compressa and Fisurella mutabilis and general surface dwellers<br />
such as <strong>the</strong> gastropods Assiminea globules, Littorina saxatilis, and Hydrobia sp. declining in<br />
abundance, while those species that burrowed predominantly in unvegetated sand, such as<br />
amphopods Urothoe grimaldi and <strong>the</strong> polychaetes Scoloplos johnstonei and Orbinia angrapequensis<br />
increased in density. Pillay et al. (2010) was also able to show that <strong>the</strong> abundance <strong>of</strong> at least one<br />
species <strong>of</strong> wading bird Terek sandpiper which feeds exclusively in Zostera beds was linked to<br />
changes in <strong>the</strong> size <strong>of</strong> <strong>the</strong>se beds, with population crashes in this species coinciding with periods <strong>of</strong><br />
lowest seagrass abundance at Klein Oesterwal. By contrast, <strong>the</strong>y were able to show that populations<br />
<strong>of</strong> wader species that do not feed in seagrass beds were more stable over time.<br />
While <strong>the</strong> precise reasons for <strong>the</strong> loss <strong>of</strong> Z. capensis beds remain speculative, <strong>the</strong> impact <strong>of</strong><br />
human disturbance cannot be discounted, particularly at Klein Oesterwal where bait collection is<br />
common (Pillay et al. 2006). By 2007 <strong>the</strong> intertidal habitat at Klein Oesterwal had been transformed<br />
from a seagrass bed community to an unvegetated sand flat which was colonized by <strong>the</strong> burrowing<br />
sandprawn Callinassa kraussi and o<strong>the</strong>r sandflat species that cannot live in <strong>the</strong> stabilized sediments<br />
promoted by <strong>the</strong> seagrass (Pillay et al. 2010). The burrowing sandprawn turns over massive<br />
quantities <strong>of</strong> sediment and once established effectively prevents <strong>the</strong> re-colonization <strong>of</strong> seagrass and<br />
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