St Helena Bay Water Quality Trust

ST HELENA BAY WATER QUALITY TRUST: St Helena Bay State of the Bay 2012 Prepared by ANCHORenvironmental St Helena Bay State of the Bay 2012 Prepared for: St Helena Bay Water Quality Trust Andre du Toit PO Box 655 Veldrif 7635 Tel: 022 7832860 Mobile: 083 2511451 Email: [email protected] Prepared by: 8 Steenberg House, Silverwood Close, Tokai 7945, South Africa Tel: 021 701 3420, Fax: 0865428711 www.anchorenvironmental.co.za Authors: K.L. Tunley, B.M. Clark and A. Biccard ANCHOR environmentalSeptember 2012 Executive summary Introduction St Helena Bay is situated on the west coast of South Africa and extends from Dwarskersbos in the north, past the Shelley Point peninsula, to Cape St Martin in the west, encompassing 18 smaller bays and the estuary of the Berg River. The Bay is positioned in the southern section of the Benguela Current System, one of four major eastern-boundary current systems which is characterised by the wind-driven upwelling of cold, nutrient rich water. St Helena Bay is positioned downstream of the Cape Columbine upwelling cell and is a retention zone for the nutrient rich water that is upwelled in this cell. The bay is subject to incidence of harmful algal blooms and regular episodes of oxygen depletion in the coastal waters, which have in the past lead to major mortality events for organisms such as rock lobsters and fish. The Bay also serves as a major node for industrial and small-scale fisheries on the west coast, as well as for other industries such as mariculture, ship repair and shipbuilding. Regular long-term monitoring in St Helena Bay was initiated by the St Helena Bay Water Quality Trust in 2001. The monitoring programme focuses on water and sediment quality and biotic indices of health, and was designed to provide an overview of trends in the health of the bay, and track to changes that may be caused by human activities. Sediment quality and benthic macrofaunal communities are monitored approximately every five years as part of this programme. The results of these assessments are used to make recommendations regarding the sustainable management of activities in the bay. This report presents results on sediment characteristics, the levels of organic material in the sediments, and the abundance and distribution of benthic macrofauna living in the sediments of the bay in 2012. It is the third assessment conducted as part of the State of the Bay monitoring program. Results from the sediment monitoring programme Sediment and benthic macrofauna samples were collected from 27 sites in St Helena Bay over a 3 day period during the month of April 2012. Samples were collected using a Van Veen grab operated from a Rigid Inflatable Boat (RIB). Samples were analysed for grain size distribution (percentage mud, sand and gravel) and organic content (Total Organic Carbon TOC and Total Organic Nitrogen TON). Assessments of the spatial distribution of different particle size groups and organic content in 2012 were conducted. These were compared with results from 2001 and 2007. Sediment samples collected in 2012 were comprised predominantly of sand, with a small proportion of mud and no gravel. Relative to data from 2001 and 2007, the 2012 results indicate a dramatic reduction in the amount of mud in the sediments and a significant reduction in the coarser gravel particle fraction in the bay since 2007. This may be associated with an observed increasing intensity and frequency of large storm events over the period 1996 to 2010. Page | i Data from 2012 also suggest that the spatial distribution of mud in the Bay has also changed dramatically over time, with areas exhibiting the highest mud content having shifted from the northerly and southerly reaches of the Bay in 2007 to the south western edge near the Berg River Estuary mouth in 2012. There have been minor reductions in the levels of organic nitrogen in the sediments at most sites since 2007, while organic carbon levels decreased in the northern reaches, increased in the vicinity of the fish factories and underwent minor changes elsewhere in the bay. The sites in the immediate vicinity of the fish factory outfalls are clearly carbon enriched compared to other areas in the bay. Effluent from fish factories is thus still making an important contribution to organic loading in the sediments but this fortunately is localised to the immediate vicinity (within 100m) of the fish factory outfalls. Carbon: Nitrogen ratios throughout the bay have increased. These changes may be associated with an increase in denitrification. Denitrification is the breakdown of organic nitrates into nitrogen and ammonia by denitrifying bacteria. These denitrifying bacteria dominate when oxygen availability is low and nitrates are available. This suggests that low oxygen conditions may have prevailed within the benthic environment of the bay since the previous 2007 survey. These conditions may be a result of variations in phytoplankton productivity associated with fluctuations in upwelling-favourable winds. These winds reached a peak in 2001 and underwent a decline up until 2006. Changes in the Southern Oscillation Index (i.e. the oscillation between El Niño and La Niña conditions) may also have played a role. Sediment conditions in the vicinity of the fish factory outfalls have deteriorated markedly since 2007. These sites are clearly carbon enriched compared to other areas in the bay, and levels of enrichment at these sites have increased disproportionately compared to other sites in the bay. This suggests that the effluent from fish factories is still having a marked effect on the organic composition of the sediments at a local scale (sites immediately adjacent to fish factory outfalls) but not at a large scale. Results from the benthic macrofauna monitoring programme All benthic macrofauna (organisms >1 mm in size) in sediment samples collected from the bay were identified to species level, counted, weighed, and assigned to major functional groups (filter feeders, detritivores, predators, scavengers, and grazers). Statistical analyses were performed on these data to assess spatial variability in the benthic macrofauna community structure and composition between sites in 2012, and to assess changes in benthic community structure over time (i.e. in relation to the 2001 and 2007 surveys). Data from 2012 suggest that the benthic macrofaunal community in St Helena Bay was dominated in terms of abundance by the polychaete Diopatra monroi and the amphipod Ampelisca anomala, and in terms of biomass, by the clam Venerupis corrugata and the polychaete D. monroi. Filter feeders were the dominant functional group in terms of biomass in 2012, while filter feeders, detritivores and predators all contributed in relatively similar proportions to total abundance. The remaining groups (scavengers and grazers) contributed relatively little to either biomass or abundance in 2012. From a spatial perspective, benthic macrofauna communities at sampling stations in the Berg estuary were significantly different from those in the rest of the bay. There were also some Page | ii significant differences among the sampling stations in the bay, with those from the eastern side of the bay being distinct from those in the northern and western sectors, and those taken from the immediate vicinity of the fish factory outfalls being distinct from one another and from other stations in the bay. Abundance of macrofauna in the stations close to the fish factory outfalls was on the whole much lower than those in the rest of the bay with the samples taken in the proximity of the factory in Stompneusbaai being completely devoid of any macrofauna. Given that the macrofauna on the west coast are typically opportunistic species, able to recover rapidly from disturbances and that the communities at two stations were in such a poor state, it is evident that effluent from these particular fish factories are having a negative impact upon the benthic macrofaunal community at these sites. However, as the results of the 2007 survey indicated, these impacts are limited to the local area around the fish factories discharge points. From a temporal perspective, the overall abundance and biomass of macrofauna in the bay decreased between 2001 and 2007 and then increased between 2007 and 2012. The overall abundance of macrofauna in the bay reached the highest levels on record in 2012, being higher than levels recorded in 2001. Changes between 2007 and 2012 were mostly associated with an increase in the abundance of polychaetes and crustaceans (most notably amphipods) and a change from a community dominated by detritivores to one with a fairly equal proportion of predators, detritivores and filter feeders. Diversity values showed an opposite pattern to that of biomass and abundance with diversity increasing between 2001 and 2007 and then decreasing again by 2012. These fluctuations in the macrofaunal community are likely to be in response to large scale natural disturbances, with few pioneering species dominating the benthos in high numbers following disturbance. Low oxygen events resulting from high levels of productivity are an example of a large scale natural disturbance that is likely to have resulted in these fluctuations in St Helena Bay. By contrast, macrofauna abundance at the stations in the estuary decreased between 2007 and 2012 and shifted from one dominated by detritivores in 2007 to one dominated by filter feeders in 2012. The cause of this was not clear though and may be related to changes in freshwater runoff to the estuary.

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