Microorganisms and Detritus in the Water Column of a Subtidal Reef of Natal

Abstract

The parameters measured in this study were light and dark-bottle uptake of NaHI4CO3, the chlorophyll content of seawater, direct counts of bacteria, microbial uptake of 14C glucose and a labelled algal extract, organic carbon in the seawater and the composition of debris washed onto the reef. The phytoplankton consisted largely of nannoplankton and primary production was low with an annual mean of P = 12.82 mg C n1r3h-', while the mean chlorophyll a content of 2.13 mg m-3 was equivalent to a dry biomass of 170.1 mg m-3 Microbial heterotrophic activity was relatively high with an annual mean glucose assimilation of V,,, = 0.612 @g l-'h-' and a total bacterial count of 2.02 X 106 ml-', equivalent to a dry biomass of 20.3 rng m-3. The largest proportion of this activity, measured by differential filtration and using antibiotic inhibitors, is performed by free bacteria as opposed to the smaller proportion conducted by bacteria attached to particles, or by microflagellates. Uptake of the labelled algal extract (V,,, -13.2-72.6 mg C m-3h-') indicates that microbial heterotrophic activity exceeds phytoplankton production. Carbon analyses gave an annual mean of 18.3 g m-3 total organic carbon, of which 20 % was particulate, the rest being dissolved organic carbon. Analysis of debris revealed that it consisted almost exclusively of seaweed, except when rivers were in flood, providing a large influx of terrestrial plant debris. These findings are compared with other published results, and their ecological significance is discussed in relation to the remainder of the environment. It is concluded that primary production by phytoplankton is not as important in supporting the biomass of filter feeders on the reef as is heterotrophic activity associated with organic detritus.

Heterotrophic utilisation of a labelled algal extract in water samples from a subtidal reef

“…The preliminary work performed in spring 1979 yielded higher values for both substrates, and values for glucose uptake measured fortnightly in the previous year (Schleyer, 1981) were also elevated (figures in parentheses in Table 1). The data for V, , from which Table 1 is derived are presented in Fig.…”

“…OR1 Reef is densely populated by the mussel Pema perna and is also inhabited by other suspension feeders, mainly the oyster Saccostrea rnargaritacea and the tunicate Pyura stolonifera, all of which are typical of this habitat in Natal. P. perna is capable of filtering particles as small as the free-living bacteria mentioned above (Schleyer, 1980b;Beny and Schleyer, 1983). Thus these bacteria probably constitute an important link in the food chain.…”

“…A study on a nearshore subtidal reef in Natal, South Africa (Schleyer, 1980a(Schleyer, , 1981, established that the major primary food source is seaweed, most of which enters the detritus food chain. Terrestrial macrophytic material is also introduced to the marine environment by rivers swollen by summer rains, making a further contribution to the detrital pool.…”

A brief overview of the ACEP project: Ecosystem Processes in the KwaZulu-Natal Bight

“…It appears that diatoms and dinoflagellates were the dominant groups, and diversity was highest in the Agulhas Current, compared to the shelf and waters farther offshore. Schleyer (1981) recorded mainly dinoflagellates in reef-associated samples collected very close inshore off Durban. Carter and Schleyer (1988) reviewed these studies, and only recently was research of this nature resumed by Barlow et al (2008), who described some aspects of the phytoplankton communities, noting that small flagellates generally dominated throughout the bight, whereas diatoms were more common in upwelling-associated, cooler waters where nutrient levels were higher.…”

“…Begg 1978). A study by Schleyer (1981) on inshore nutrient sources provided an indication of the importance of terrestrial detrital inputs in a nutrient-poor part of the shelf, and Lamberth et al (2009) demonstrated the importance of Thukela River flow to bight fisheries. However, studies on offshore ecosystem processes such as primary production and nutrient cycling have been hampered by a lack of access to research vessels, particularly since the physical oceanographic research in the 1960s, and given the loss to the research community of the RS Meiring Naudé in the 1980s.…”