Macrobenthos used to validate multi-criteria derived marine biodiversity spatial zones in KwaZulu-Natal, South Africa.

Abstract

Compared to terrestrial ecosystems, the characteristics of marine ecosystems remain largely under-explored. Marine and coastal ecosystems provide a number of ecological services and societal benefits (resources for commercial opportunity, food, recreation, and transport) which, in turn, has developed a strong reliance on these ecosystems. However, the increasing direct extraction of living and non-living resources and effects of urbanisation of adjacent coasts has placed a significant loss of habitats and associated essential diversity. To conserve biodiversity and retain specific goods and services provided by these ecosystems, marine conservation plans aim to protect spatial areas that are critical in the support of these benefits. Due to the paucity of adequate biological data and the prohibitive cost of directly sampling benthic biota over large areas, the most effective means of developing benthic habitat maps, used as biodiversity surrogates, is to use commonly available marine abiotic attributes. In KwaZulu-Natal (KZN), through marine spatial planning (MSP) the derivation of a marine conservation plan is well underway. The next step is to expedite the plan by investigating whether surrogates for biodiversity exist at different ecosystem levels, one being the infauna of unconsolidated sediments, mid-shelf 50-80 m. This work presents an outcome of the ACEP ‘Surrogacy Project’ that assessed whether predefined biodiversity zones (biozones) represent the taxonomic/functional attributes of macrobenthic communities. Biozones were subdivided into various subclusters from Richards Bay to uMkhomazi with 19 (57 replicates) stations sampled during the winter of 2014 across the biozones to represent replicate ‘treatments’. Macrobenthic communities were classified taxonomically, to the lowest level possible, and then on biological traits. Community patterns were investigated along the mid-shelf, and related to measurable biophysical factors. Environmental parameters measured included sedimentary characteristics as well as the bottom 5 m of water column characteristics per station. A total 33 215 individuals belonging to 634 taxa were recorded along the mid-shelf, of which the majority were Polychaeta and Crustacea, with the latter being highly abundant. Cluster analysis resolved into seven taxonomic groups distributed according to different habitats that are characteristic to the KZN shelf. The use of coarser taxonomic resolution (Phylum-Genus) or indicator taxa (Polychaeta and Amphipoda) as surrogates for total community richness were independently investigated using the same macrofaunal abundance data. Results showed similar clustering of samples to total fauna (Species-level) when data were analysed at Family-Genus taxonomic level and at Polychaeta indicator taxa, suggesting that the same amount of information was being gained using data based on these taxonomic level and indicator taxa. The results of the BIOENV analyses were also broadly similar for both taxonomic levels of analyses, in terms of both the proportion of the variation in assemblage structure explained by the selected environmental variables and the choice of selected variables. These results suggested that the information gathered at Family-Genus level and Polychaeta indicator can be used as a proxy for the whole macrobenthic community. This has important implications for future studies and for MSP. Using nine traits, across 51 categories, four main functional groups were found off Thukela, Zinkwazi to Durban, and Durban to uMkhomazi. The groups were characterised as being free-living carnivores, hard-skeleton direct-developing omnivores, and soft-bodied or hard-shelled omnivores with planktotrophic larvae. These patterns were explained by the KZN shelf habitat complexity, including level of different sediment grains, TOC, carbonates, water column turbidity, salinity, dissolved oxygen and temperature. Thus far, distribution patterns and functional attributes of the macrobenthos do not fully agree with modelled biozone separations (KZN MSP biozone model). Because they are an important component of marine ecosystem functioning, biozone model derivations require the addition of a macrobenthic component, in particular information about diversity patterns, to identify areas for conservation. Suggested, is a refinement of the current benthic habitat layer by incorporating biological data. Further, by using validated sediment distribution, taxonomic and functional attributes that determine soft-bottom macrofaunal distribution at a variety of spatial scales, an alternative biozone model to the current MSP predefined biozones was proposed. This multi-approach resolved into a simplified model with four biozones. These are likely better predictors of spatial variation in ecosystem processes and biodiversity as domains that are biologically informed, and are a key requirement for effective marine management. This study demonstrates the critical importance of testing assumptions about surrogacy and an approach for refining surrogates. Further studies are required to establish whether the proposed model adequately represents other ecological components (e.g. epifauna). The findings of this study contribute significantly to existing local knowledge, including augmenting and refining taxonomic information of the KZN shelf. In addition, this study subsidises poor information for large spatial areas in local and national marine conservation plans. The proposed biozone model may facilitate an understating of ecosystem process in the region and contributes to integrated marine management.