Spatial variability of macro-benthic invertebrate assemblages in the Kogelberg region, focusing on the Betty’s bay marine protected area

Abstract

There is large variation in the patterns of biological diversity in the ocean. This variation is a result of the range in combinations of physical and biological interactions which exist in the marine environment. The South African marine environment supports a diversity of marine ecosystems in which species richness and endemism have been found to be high (33 % endemism). The high diversity of organisms promotes the maintenance of healthy and ecologically stable ecosystems. The diversity of life in the oceans however, is under threat as a result of numerous natural and anthropogenic threats such as over-fishing, over-extraction of marine resources, ocean acidification, to name a few. Marine Protected Areas (MPAs) have been implemented throughout the world to address the threats facing the marine environment. Marine Protected Areas are considered the most effective tool at protecting marine ecosystems and play a key role in marine Ecosystem Based Management (EBM). Marine Protected Areas also offer the added benefit of providing areas in which research can be conducted, more specifically in the area of conservation, MPAs have been declared for either fisheries management purposes, biodiversity conservation, or for management of conflict between competing users. The Betty’s Bay MPA is located within the Kogelberg Biosphere Reserve (KBR) on the south-west coast of South Africa. The MPA was established in 1990 as the H.F Verwoerd Marine Reserve (re-proclaimed as the Betty’s Bay MPA in 2000) to protect, among others, the endangered African penguin (Spheniscus demersus), abalone (Haliotis midae) and the west coast rock lobster (Jasus lalandii). To date, few studies have been conducted within the area and as a result, knowledge of the sessile macro-benthic flora and invertebrate fauna of the Betty’s Bay MPA is sparse. The paucity of information of the macro-benthos creates a barrier for informed management decisions of the MPA, as well as a proposal to expand the MPA boundary. To aid in an assessment of and provide information on the macro-benthic flora and invertebrate fauna assemblage patterns and diversity within the area, a spatial photo-quadratic survey was conducted in November 2012 and January 2013. A photo-quadrat of 0.33 m2 was used to survey the MPA and outside its boundary at two depth categories (10 m – 15 m and 20 m – 25 m). Representative samples of specimens were randomly collected within the sampling stations to create a database of the species occurring within the area and in so doing, aid in species identification and verification during photo-quadrat analysis. A total of 881 viable (clear with focal frame correctly positioned) photographs from 10 sampling stations were analysed using a grid overlay of 10 x 10 (100) points per photograph. A 10 x 10 grid was used to allow for percentage cover and individual (for non-colonial species) or colony (for colonial species) abundance estimates of faunal organisms, to be calculated simultaneously. Organisms occurring under each point of the grid were identified and the individual or colony abundance of faunal species and percentage cover per species for each sample was recorded. Species accumulation curves and a rarefaction model, the Morgan-Mercer-Flodin (MMF) Model, were used to assess the sampling effort and provide the number of faunal species estimated to be within the study area, respectively. Uni- and multivariate statistical procedures were used to analyse percentage cover estimates from photo-quadrats and compare the floral, sessile and semi-motile macro-benthic community structure along a spatial and bathymetric gradient within the study area. The univariate indices selected for analyses and interpretation of percentage cover data included species richness (S), percentage cover (%), Pielou’s evenness index (J’) and the Shannon-Wiener diversity index (H'). Data met the assumptions for parametric tests and multi-effects Analysis of Variances (ANOVAs) were conducted to assess whether depth or location had a greater effect on variances of species diversity, richness and percentage cover between samples. Cluster and ordination analyses were used for multivariate comparisons. Results showed that species accumulation curves for the study area did not reach an asymptote and a higher estimated than observed faunal species richness was calculated for the area, suggesting that sampling effort was not sufficient. Despite this, species accumulation curves indicated that 140 to 168 photographs per station (curves began levelling off at this amount) were enough to obtain an adequate estimate of species richness within the study area. This suggests that too few photographs were collected for most stations. However despite this, 76 % of the faunal species estimated to occur within the study area were recorded, indicating that the area was adequately sampled. The mean species richness measured S = 15.49 ± 4.55 SD, n = 881 samples and the mean species diversity was average (H' = 2.21 ± 0.48 SD, n = 881 samples) within the study area. The mean percentage cover of benthic organisms in the area was 78.03 % (± 11.80 %, n = 881 samples). In addition, the area appears to be fairly even with a mean evenness of 0.88 (± 0.07 SD, n = 881 samples), indicating that the percentage coverage of individuals was well distributed amongst the species within the area. The multi-effects ANOVAs revealed that location had a greater effect on the variances of samples for species diversity and species richness than depth. Upon further investigation it was found that the species richness and diversity of samples inside the MPA were significantly higher (p < 0.001) than that outside the MPA. No significant differences in percentage cover were found to occur between locations (p > 0.05), however the mean percentage cover of most taxa was higher outside the MPA. This difference between diversity inside and outside the MPA could be as a result of a number of factors. Habitat heterogeneity, found to be high in previous studies conducted in the Betty’s Bay area, may be an example of one of such factor. Findings suggest that the MPA is potentially fulfilling its management objective of biodiversity conservation for the species included in this particular study. Depth was the primary factor affecting percentage cover within the study area, with deep samples having significantly lower (p < 0.01) percentage cover estimates than shallower samples. The multivariate analysis as well as an Analysis of Similarity (ANOSIM) using depth as a factor, also indicated differences in communities. Algae dominated (i.e. had the greatest percentage cover) areas in the shallower-reaches. The percentage cover of Porifera, Bryozoa and other sessile filter- and suspension-feeders increased with increasing depth, thus, revealing a community shift similar to other subtidal assemblages on hard substrata. It was concluded that macro-benthic assemblages within the Betty’s Bay area are mainly influenced by depth. Colonial taxa such as Cnidaria and Algae were dominant in the study area. Dominant species in the area included the cnidarian species Eudendrium spp 1 and algal species Leptophytum foveatum, Jania adhaerens, Rhodophyllis reptans and Rhodymenia obtusa. The dominance of Algae in relation to other taxonomic groups in the area is important to monitor as it may influence the settlement of benthic invertebrate macro-fauna and increase the abundance and distribution of epifauna and infauna, for example, Eudendrium spp 1. It is also important to prioritize the effective monitoring and management of the increased abundance of the carnivorous west coast rock lobster, Jasus lalandii which reduces herbivore abundance and causes a subsequent increase in Algae. Further investigations of benthic invertebrates within the study area, which take environmental variables and habitat heterogeneity into account, need to be conducted in order to establish whether the differences in diversity observed during this study can be linked to protection. Should the significant differences be a result of protection within the MPA, managers should consider expanding the MPA or, failing that, converting the MPA to a Category 1 or “no-take” MPA as population fluctuations in exploited organisms, may affect populations of other unexploited species. The expansion of the MPA or conversion to a Category 1 MPA may ensure that the marine benthic biodiversity within the area is well-conserved. Moreover, a monitoring programme examining both biotic and abiotic factors should be implemented within the area. Within the monitoring programme, managers should attempt to have organisms identified to genus level at the very least. A monitoring programme of this nature would aid managers to be more informed and proactive and implement an ecosystems-based approach to management.