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    An investigation into the ecology and biogeography of subtidal seaweed communities of KwaZulu-Natal
    (2005) Evans, Adrian; Bolton, John; Anderson, Rob
    An investigation into the ecology and biogeography of subtidal seaweed communities of KwaZulu-Natal. This study is the first to investigate the ecology and biogeography of subtidal seaweed communities along the coast of the province of KwaZulu-Natal, on the east coast of South Africa. Intertidal studies have found a biogeographical break in the region of Cape St. Lucia and it was hypothesized that subtidal communities would mirror these trends. North of Cape St. Lucia, the continental shelf is narrow and the warm Agulhas current brings warm tropical waters onto the coast. South of St. Lucia, the continental shelf widens into the Natal Bight which extends for 160 km to Durban. Topographically-induced upwelling brings cooler nutrient-rich waters onto the Bight which is often discoloured by run-off from rivers like the Tugela. The Bight is influenced by wind driven currents and water is retained for extended periods. The high sedimentation and poor light conditions make this region poor for algal growth. A total of 15 samples were obtained from 10 sites along the coast between 1 and 30 m deep. Each sample comprised a number of25 x 25 cm quadrats [usually five] that were scraped off hard substrata. A number of easily measured environmental variables were recorded for each quadrat. These were correlated against the biological findings to investigate the effects on algal communities. Temperature recorders were placed at 15 mat Sodwana, Aliwal and Ballito. A year's temperature data was recorded and monthly means calculated. Biogeography was assessed using ordinations [Detrended Correspondence Analysis] and hierarchical clustering analyses [TWINSPAN]. A remarkable species richness [294 species] was recorded in 71 quadrats [a total sampling area of 4.44 m 2 ], with a maximum of 40 species being recorded in a single quadrat. Crustose corallines and blue-green algae were not included in this analysis. The species were dominated by the Rhodophyta [77.9% of total species] with the remainder of the species from the Phaeophyta [9.9%] and Chlorophyta [12.2%]. The total biomass of2420.8 g was more evenly distributed, with Rhodophyta [39.6% of total biomass] and Phaeophyta [35.2%] contributing similar amounts and the remainder coming from the Chlorophyta [25.2%]. The Phaeophyta included a number of genera [e.g. Zonaria, Dictyota, Padina and Lobophora] that are known to be chemically defended against herbivory and able to attain larger sizes. The Chlorophyta also contribute a higher percentage of biomass than species numbers and this is mainly due to the large sand-binding species, Caulerpa filiformis, which dominated the heavily sand-affected Zinkwazi site. The samples were dominated by algal turfs, with a few larger algae interspersed. Algal turfs [interwoven species forming mats less than 3 cm high] are common throughout the world and dominate algal communities in the tropics. They are morphologically similar but taxonomically diverse as can be seen by the high species richness found. There was a high percentage of rare species with 76.5% of the species being recorded in less than 7% of the quadrats and 64.6% of the species having a total biomass of less than 1 g. Similarity calculations showed low average values between quadrats of the same sample [30-50%] indicating extremely high spatial variability in communities and a need for more extensive sampling. However due to the difficulties of collecting, sorting and identifying turf algae this was logistically impossible in this study, and is the main reason why turf algae are often ignored or simply lumped as a form group in many studies. This study shows that in KwaZulu-Natal these turfs are species-rich and highly diverse and their floristic identities cannot be ignored in any meaningful ecological study. Depth was found [using regression analyses] to be the most important factor affecting the seaweed communities with species number and biomass declining significantly with depth. This is directly attributed to the decrease in light availability at deeper depths. Species composition and biomass were found to be highly variable in the first 5 m and generally with higher biomass and species number than the deeper samples. The percentage of calcified species' biomass also decreased with increasing depth. This is attributed to the combination of increased water motion from wave action increasing nutrient availability but causing mechanical damage. This results in a highly heterogeneous zone[< 5 m] that favours highly productive and fast growing species. A species number and biomass regression revealed no pattern. A small amount of biomass in the form of turf algae could result in many species and larger species contributing high amounts of biomass did not exclude turfs through shading or mechanical damage. The crustose corallines did not increase with depth in response to the reduced algal cover with depth. The crusts require high levels of herb ivory to create a habitat, but still require sufficient light and are particularly susceptible to shading from larger species. Distance to sand did not have an effect on the species composition or richness. This measurement was used to assess the effects of sand scour but the mechanisms for this are more complex and intricate than just distance to sand. North of the Natal Bight, the temperature of Sodwana and Leadsman were highly similar for the entire year with annual mean temperatures of 24.2 °C and 24.1 °C, respectively. These sites are tropical by definition with the mean monthly average never below 20 °C. Ballito situated in the centre of the Natal Bight had temperatures of approximately 2 - 3 °C lower than north of the bight and an annual mean temperature of 22.2 ° C. The monthly temperature did drop below 20 °C for August and is therefore not tropical by definition. As temperature is the most important factor controlling global phytogeographic provinces, these data support the intertidal findings of a biogeographic break in the region of St. Lucia Seaweed biogeographic patterns are generally based on the distribution of species along a coastline on the basis of presence-absence data of the total species composition. Thus the sampling method used in this study is not ideal for biogeographic analysis, but was necessary due to the turf algal component; however some inferences can still be made. The ordinations [CCA] and hierarchical clustering [TWINSP AN] analyses grouped the quadrats from sites north of the Bight closely together and separate from the sites in and south of the Bight. Thus there are good indications that the biogeography of some of the subtidal seaweeds of KwaZulu-Natal shows a similar pattern to the intertidal seaweeds: the species in the sites north of St. Lucia coming from the southern extension of the large Indo-West Pacific flora and thus being grouped together due to the similar species compositions; and the species in southern sites occurred in an area where a rapid change of species turnover is occurring and is often placed in two separate groups by the analyses.