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

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Top 10 species contributing to similarity State of Saldanha Bay & Langebaan Lagoon 2011 Benthic macrofauna Table 7.3. Top ten species characterising the benthic macrofaunal communities in Central Big Bay and Langebaan Lagoon in 2011. Group (Similarity) B (55%) Big Bay central C (50%) Langebaan Lagoon Species Common name/group Species Common name/group Iphinoe africana Cumacean (Crustacean) Ampelisca palmata Amphipod Ampelisca spinimana Amphipod Ostracoda B (smooth) Ostracod Sabellides luderitzi Polychaete Hymenosoma orbiculare Crown crab Nassarius vinctus Dog whelk Ostracoda A (spiky) Ostracod Photis longidactylus Amphipod Maldanidae sp. A Polychaete Nephtys sphaerocirrata Sand worm Thaumastoplax spiralis Three-legged crab Scolaricia dubia Polychaete Anemone Anemone Nephtys hombergi Sand worm Notomastus latericeus Polychaete Nassarius speciosus Purple-lipped dog whelk Paraphoxus oculatus Amphipod Tellina gilchristi Gilchrist's tellin Orbinia angrapequensis Polychaete 140 ANCHOR e n v i r o n m en t a l
State of Saldanha Bay & Langebaan Lagoon 2011 Benthic macrofauna 7.4.1.2 Temporal Analysis Small Bay The suspension feeding sea-pen communities, which were reported in Small Bay in 1975, have recovered. Filter feeders have remained a dominant functional group; however, this group is made up mostly of the opportunistic mud prawn (Upogebia capensis) and smaller amphipod species belonging to the Ampelisca genus. In all surveys since 1999 detritivores have also been a dominant functional group, even more so in some years than filter feeders. Most notably, the 2008 survey revealed a dramatic reduction in the proportion of filter feeders and increase in the proportion of detritivores. The dominant detritivores included tongue worms (Ochaetostoma capense) and polychaetes belonging to the genera Polydora and Euclymene. This dramatic shift to a detritivore dominated benthic ecosystem, seen in 2008, can be attributed to an increased deposition and accumulation of fine particles and organic matter between 2004 and 2008 (See §5.2.1 for more details on this). This in turn, can be attributed to the restricted flow, altered wave energy, deposition of fine sediments and increased organic matter, which resulted from harbour construction and fish factory, mussel farm and sewage effluents. In all years since 2008, filter feeders have been the dominant functional group both in terms of abundance and biomass. The 2011 survey revealed a substantial increase in the numbers of detritivores. This was not matched in terms of biomass indicating an increase in small detritivorous species such as the polychaetes belonging to the genus Polydora. Crustaceans have dominated the benthic macrofauna in terms of biomass and abundance in all surveys conducted since 1999. The 2008 survey revealed that there had been a drastic reduction in the overall biomass and abundance of benthic macrofauna in Small Bay. This was most likely a result of the dredging activities conducted at the Mossgas quay and the Multi Purpose Quay in 2007/08. Much of the reduction in biomass in 2008 could be accounted for by the reduced biomass of crustaceans, however the abundance of crustaceans did not decrease in the same manner. This indicates that many small crustaceans (most likely r-selective) dominated the benthic community following dredging. Since 2008 the average biomass in Small Bay has been increasing. This increase in biomass can be principally accounted for by the increased biomass of crustaceans and tongue worms (Echiuroidea) between 2008 and 2011. Interestingly, the abundance of crustaceans declined between 2008 and 2009, while the biomass increased. This suggests that the community had shifted from one composed primarily of small, opportunistic crustaceans to one composed of fewer, larger, (most likely K-selective) crustaceans in 2009. Small (low biomass) 1 polychaetes increased substantially in abundance between 2008 and 2009, then declined again in 2010. This suggests that the polychaetes were able to compete with small opportunistic crustaceans and colonise the recently disturbed benthic habitat between 2008 and 2009. It is likely that the polychaetes were then outcompeted between 2009 and 2010 by the growing populations of larger crustacean species. This is a possible indication of the succession in benthic macrofauna communities following the 2007/08 dredging. Other signs of the recovery of the system evident from the 2010 and 2011 surveys include the increase in the average biomass and abundance of gastropods and bivalves between 2008 and 2011. However, the 2011 survey revealed a substantial increase in the abundance of polychaetes. This increase was not reflected by the biomass results, indicating that once again the Small Bay sites had been colonised by small polychaete species. The reasons for this 1 This is evident given that the overall biomass of polychaetes did not increase substantially while the abundance did. These are most likely small, fast growing r-selected species. 141 ANCHOR e n v i r o n m en t a l
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