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

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State of Saldanha Bay & Langebaan Lagoon 2011 Benthic macrofauna variations in environmental conditions which may result from either natural factors or anthropogenic activities. In stable environments the community composition is controlled predominantly by biological interactions rather than by fluctuations in environmental conditions. Species found in these conditions are known as “K-strategists” and are selected for their competitive ability. K-strategists are characterised by long-life spans, larger body sizes, delayed reproduction and low death rates. Intermediate communities with different relative proportions of opportunistic species and K-strategists are likely to exist between the extremes of stable and unstable environments. The statistical program PRIMER 6 (Clarke and Warwick 1993) was used to analyze the benthic macrofauna data. Data were root-root (fourth root) transformed and converted to a similarity matrix using the Bray-Curtis similarity coefficient. A cluster analysis was performed in order to find ‘natural groupings’ between samples (sites). The results of the cluster analysis are displayed on a dendrogram which graphically displays the similarity of sites by grouping the sites. Statistically significant clusters of sites are revealed using a SIMPROF analysis. These results were plotted geographically using ArcGIS to reveal any spatial trends in the sites grouped according community composition similarity. SIMPER analysis was used to identify species principally responsible for the clustering of sites. These results were used to characterise different regions of the system based on the communities present at the sites. It is important to remember that the community composition is a reflection of not only the physico-chemical health of the environment but also the ability of communities to recover from disturbances. 7.3.2.2 Diversity Indices A number of indices (single numbers) can be used as measures of community structure; these include the total number of individuals (N), total number of species (S), the total biomass (B), and the species equability or evenness, which is a measure of how evenly individuals are distributed among different species. Diversity indices provide a measure of diversity, i.e. the way in which the total number of individuals is divided up among different species. Understanding changes in benthic diversity is important because increasing levels of environmental stress generally decrease diversity. Two different aspects of community structure contribute to community diversity, namely species richness and equability (evenness). Species richness refers to the total number of species present while equability or evenness expresses how evenly the individuals are distributed among different species. A sample with greater evenness is considered to be more diverse. It is important to note when interpreting diversity values that predation, competition and disturbance all play a role in shaping a community. For this reason it is important to consider physical parameters as well as other biotic indices when drawing a conclusion from a diversity index. The following measures of diversity were calculated for each sampling location using PRIMER V 6: The Shannon-Weiner diversity index (H’): H’ = - Σ i p i (log p i ) (1) Where pi is the proportion of the total count arising from the ith species. This is the most commonly used diversity measure and it incorporates both species richness and equability. The Pielou’s evenness index (J’): J’ = H’ observed / H’ max (2) Where H’max is the maximum possible diversity which would be achieved if all species were equally abundant (= log S). This is the most common expression of equability. The Margalef’s index (d) of species richness: D = (S-1)/ log N (3) Where S is the total number of species and N is the total number of individuals. 132 ANCHOR e n v i r o n m en t a l
State of Saldanha Bay & Langebaan Lagoon 2011 Benthic macrofauna Species richness is often simply referred to as the total number of species (S), but this is very dependent on sample size. The Margalef’s index thus incorporates the total number of individuals (N) and is a measure of the total number of species present for a given number of individuals. The diversity (H’) value for each site was plotted geographically and this was used to interpolate vales for the entire system using ArcGIS in order to reveal any spatial patterns. The average diversity value was also then calculated for three pre-designated locations (Small Bay, Big Bay and Langebaan Lagoon) for 1999, 2004, 2008, 2009 and 2010. In order to test if the observed changes in diversity were statistically significant (p
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