State of the Bay Report 2010-Final - Anchor Environmental

7.1 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 7.2).
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 to 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 7.2). 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
State of the Bay 2010: Saldanha Bay and Langebaan Lagoon 149