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

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State of Saldanha Bay & Langebaan Lagoon 2011 Intertidal invertebrates 8.3.2 Spatial Variation in Community Composition Figure 8.10 illustrates biotic cover, species number, evenness, and Shannon-Wiener diversity indices at the eight rocky shore sites (indices are calculated for the whole shore across all zones). Foremost it is apparent that the amount of rock surface covered by animals and seaweeds steadily increased with increasing wave exposure, with the exception of North Bay, where great parts of the very smooth rock surface was devoid of intertidal life. The two very sheltered boulder beaches in Small Bay were generally impoverished with little biotic cover and lowest species numbers, whereby Dive School had on average twice as many species as Jetty. There is a certain trend of increasing species richness with greater wave exposure with the highest species count at Marcus Island. In contrast, Dive School and Jetty had highest evenness. This indicates that the communities were not dominated by one or few species but rather all species were more or less equally abundant. Evenness reduced towards semi-exposed sites but increased again at greater exposure levels. A similar picture is evident for the Shannon-Wiener diversity index. Lowest evenness and diversity were found at Iron Ore Terminal. A low evenness means that the biota is dominated by one or few species, which at Iron Ore Terminal is clearly the invasive barnacle B. glandula. The abundances (as opposed to the space they occupy on the rock surface specified as percentage cover) of the most common mobile species per site are illustrated in Figure 8.9. Only few mobile species occurred at the high shore, the most prominent being the typical high shore species Oxystele variegata at the very sheltered sites and the periwinkle Afrolittorina knysnaensis. The mid shore had a greater array of common mobile species: O. variegata was still relatively common at very sheltered mid shores, as was O. tigrina. Whereas Siphonaria serrata and C. granatina were present at nearly all mid shores, Scutellastra granularis and A. knysnaensis were more abundant at semi-exposed to exposed sites. A. knysnaensis is normally abundant primarily in the upper intertidal where it congregates in crevices to escape the heat of the day, while emerging at night or on moist days to feed (Branch et al. 2010a). Particularly at Iron Ore Terminal, however, this snail was also abundant at the mid shore where it lives amongst the barnacle B. glandula (Figure 8.10). For a rocky shore in Table Bay, it has been shown that the abundance of A. knysnaensis is strongly positively correlated with that of B. glandula (M. Van Zyl, University of Cape Town, unpublished data 2009 cited in Griffiths et al. 2011). Laird & Griffiths (2008) also found a very noticeable difference between barnacle invaded and non-invaded areas reporting that A. knysnaensis were more abundant, and extended farther down the shore, in invaded areas where they nestled between dense colonies of B. glandula. The study demonstrated positive correlations for all shore heights. It is suggested that the barnacle cover increases habitat complexity and provides shelter for the periwinkles from strong wave action. For the eight study sites in Saldanha Bay, such positive relationship is, however, not that conclusive when the data across all shore heights are included. Although the correlation is statistically significant, there are many situations in the high shore were B. glandula were absent but periwinkles plentiful (Figure 8.11). However, confining the analysis to the mid-shore, were the alien barnacle thrives particularly well and is densest, the positive relationship is evident (Figure 8.11). In the high shore where wave stress is minimal, the periwinkle is naturally abundant; in the mid-shore, however, wave stress increases and without shelter, the periwinkle normally declines in abundance. This would suggest that A. knysnaensis abundance is independent of the barnacle’s presence in the highest intertidal but lower down the shore, the barnacle matrix offers refuge and/or greater substrate complexity for the periwinkle to extend its range lower down the shore. 174 ANCHOR e n v i r o n m en t a l
State of Saldanha Bay & Langebaan Lagoon 2011 Intertidal invertebrates Cover% Shannon-Wiener 100 90 80 70 60 50 40 30 20 10 0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 Dive School Jetty Schaapen East Schaapen West Iron Ore Jetty Lynch Point North Bay Marcus Island Evenness 36 34 32 30 28 26 24 22 20 18 16 14 12 0.90 Species Number 0.85 0.80 Mean Mean±SE 0.75 Mean±SD 0.70 0.65 0.60 0.55 0.50 Dive School Jetty Schaapen East Schaapen West Iron Ore Jetty Lynch Point North Bay Marcus Island Mean Mean±SE Mean±SD 1.4 Dive School Jetty Schaapen East Schaapen West Iron Ore Jetty Lynch Point North Bay Marcus Island 0.45 Mean Mean±SE Mean±SD Dive School Jetty Schaapen East Schaapen West Iron Ore Jetty Lynch Point North Bay Marcus Island Mean Mean±SE Mean±SD Figure 8.8. Box & whisker plots of per cent cover, species number, evenness and Shannon-Wiener diversity at the eight rocky shore sites. Sites are sorted from left to right according to increasing wave exposure. 175 ANCHOR e n v i r o n m en t a l
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