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

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State of Saldanha Bay & Langebaan Lagoon 2011 Benthic macrofauna mussel M. galloprovincialis began establishing dense intertidal beds on two intertidal sand flats close to the mouth of Langebaan Lagoon (Hanekom and Nel 2002). The mussel beds reached an estimated biomass of close to eight tons in 1999 raising concerns that the invasion could spread to the rest of the lagoon and other sandy substrata (Hanekom and Nel 2002). A comparative study between invaded and non-invaded areas showed a replacement of sandbank species communities by those typically found in rocky shores where the mussel provided the hard substratum suitable for their settlement (Robinson and Griffiths 2002). In early 2001, however, the mussels had started to die off and by mid-2001 only dead shells and anoxic sands remained. The precise causes of the die off have not been established but siltation and lowered food availability are suggested as possible reasons behind the declines (Hanekom and Nel 2002). In an effort to prevent the re-settlement of the mussel South African National Parks began to remove dead mussel shells in late 2001 (Robinson et al. 2007b). A study looking at the ecological impacts of the invasion and subsequent clearing of the dead shells was done comparing pristine non-invaded areas, invaded areas that had living mussel beds, un-cleared areas with no living mussels but a thick remnant mussel shell layer, and areas cleared of dead mussels (Robinson et al. 2007). The study found that community composition differed significantly between non-invaded and invaded areas where mussel created a multilayered complex habitat promoting the colonization of rocky-shore species. This significantly increased biomass but not species diversity, reflecting a replacement of the natural sandy ecosystem for a typical rocky-shore system (Robinson et al. 2007). After the die-off and subsequent clearing of the dead shell remains, some recovery was already evident between non-invaded and cleared areas after only 5 months. Although no significant differences were found between non-invaded and cleared areas, the absence of more than 50% of the species from the cleared areas shows that total recovery had still not been attained. The mussel invasion thus dramatically altered natural community composition which remained different from non-invaded areas even 5 months after the clearing, when the study ended. Fortunately this invasion was short lived. The overall abundance and biomass of macrofauna in Langebaan Lagoon declined sharply again between 2004 and 2008. The 2008 survey also indicated that the proportion of filter feeders had been drastically reduced. These results were possibly linked to the dredging that took place at the northern end of lagoon as part of the beach erosion mitigation. The biomass then almost doubled between 2008 and 2009, principally owing to a marked increase in crustaceans. The abundance of macrofauna did not increase proportionately suggesting that larger-bodied crustaceans colonised the lagoon between 2008 and 2009. There were further increases in the abundance and biomass of benthic macrofauna between 2009 and 2010. The 2011 survey revealed that the abundance of small (low biomass) polychaetes had increased in the Lagoon, while the overall biomass of crustaceans had increased. In addition bivalve communities had increased both in terms of abundance and biomass. The overall biomass measured in 2010 and 2011 exceeded that measured in 1975, however the diversity of taxa has been reduced and crustaceans overwhelmingly dominated the benthic macrofauna biomass. This suggests that the Lagoon may have undergone an ecosystem shift. The 2011 survey results suggest that the Lagoon is in a relatively healthy state given the increases in biomass and abundance and relative stability of functional groups. However, similar to that seen in Saldanha Bay, there had been an increase in the abundance of small polychaetes. The results of the sediment survey in 2011 also revealed system-wide reduction in the mud content and increases in the concentrations of some trace metals. The sediment results coupled with the system wide trends seen in the benthic macrofaunal communities certainly suggest a system wide perturbation, the source or cause of which is unclear. The average diversity of macrofauna in Langebaan Lagoon was relatively low in 2004, but increased between 2004 and 2008 and 2008 and 2009 such that the lagoon supported a moderate level of diversity. The 2010 survey revealed that there had been a very slight decrease in the diversity of macrofauna. By 2011 the average diversity of benthic macrofaunal communities in the Lagoon had increased to a fairly high level of diversity. The area is subject to natural disturbance (strong currents and tidal variation) and under such conditions the diversity of benthic communities 158 ANCHOR e n v i r o n m en t a l
State of Saldanha Bay & Langebaan Lagoon 2011 Benthic macrofauna is expected to fluctuate slightly, be relatively patchy and remain moderate to high. The overall increase in diversity since 2004 certainly suggests that the communities have been in a state of recovery following one major perturbation. This recovery seems to vary spatially as diversity values are very patchy around the Lagoon. The Lagoon comprises a system of shallow sand bars and deeper channels which are subject to strong currents and tidal activities. This ongoing natural disturbance varies spatially and temporarily depending on sediment dynamics within the lagoon. The high diversity levels recorded at many of the sites may be a result of intermediate levels of disturbance which would allow for communities to pass the pioneering phase and increase in diversity but without reaching a stable state. The areas with low diversity may be a result of high levels of disturbance selecting for a few opportunistic species. 7.10 Summary of benthic macrofauna findings A range of benthic community health indicators examined in this study over the period 1999 to 2011 has revealed that benthic health most likely deteriorated in Small Bay from 1999 to 2008, but has recently (since 2009 survey) started to show signs of recovery. Benthic health within Big Bay improved marginally between 1999 and 2008 after which it decreased again to a state similar to that observed in 1999. There has been little change in benthic health within Langebaan Lagoon over the last decade. Small Bay and Big Bay have both suffered a significant reduction in species diversity over the last decade, although Small Bay, and in some cases Big Bay, is showing signs of recovery. Most notable is the return of the suspension feeding sea-pen Virgularia schultzei to Big Bay since 2004 as well as an increase in the percentage biomass of large, long lived species such as the tongue worm Ochaetostoma capense, and several gastropods. The most severely impacted sites within Small Bay in 2011 remain the Yacht Club basin and the Multi-purpose Quay. These sites are prone to the accumulation of pollutants due to restricted water movement in these areas. Benthic fauna have been almost entirely eliminated from the Yacht Club basin in Small Bay, which is also the site registering the highest concentrations of metals and other contaminants. Although benthic health within Small Bay is showing signs of improvement, the health status of this site is still lower than that of Big Bay and Langebaan Lagoon. In order to ensure the continued improvement in the health of the Small Bay marine environment it is recommended that stringent controls are placed on the discharge of effluents into Small Bay to facilitate recovery of benthic communities in this extremely important area. 159 ANCHOR e n v i r o n m en t a l
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