Seasonal dynamics of meiofauna in a South African temporarily open/closed estuary (Mdloti Estuary, Indian Ocean)

Abstract

One hundred and eighty-four of the 250 estuaries in South Africa are currently classified as temporarily open/closed and close off from the sea during the dry season, under low river inflow. The subtropical Mdloti Estuary, on the KwaZulu-Natal north coast, is one of these systems and hardly any information is available on its meiofauna. The abundance, biomass, composition, and grazing impact of meiofauna, as well as the key environmental factors that affect these variables, were investigated with emphasis on the contrast between open and closed phases of the estuary. Microphytobenthic chlorophyll a concentrations varied between 1.4 to 480 mg m⁻². Meiofauna were composed of nematodes, harpacticoid copepods, crustacean nauplii, mites, turbellarians, polychaetes, oligochaetes, ostracods and chironomids. Total abundance of meiofauna showed large variability both spatially and temporally and ranged from 0.4 to 88×10⁴ ind. m⁻². Nematodes, mites and harpacticoid copepods occurred more often than other groups in the sediment. Total meiofauna carbon biomass exhibited similar temporal as well as spatial patterns as abundance and varied from 0.5 to 440 mg C m⁻². A carbon-based grazing model, applied to the total meiofauna, provided estimates of potential daily ingestion rates ranging from 1.8 to 857 mg C m⁻². Nematodes, mites and harpacticoid copepods contributed the most to the total potential daily ingestion rate of meiofauna in the Mdloti Estuary. Potential ingestion rates, determined using allometric equations, showed that meiofauna consumed from 0.1 to 254% of the microphytobenthic standing stock. Overall, meiofauna were likely not food limited and grazing on microphytobenthos was low, averaging 11% for the whole survey. A principal component analysis, applied to the whole study area and sampling period, indicated that major variations in meiofaunal community are mainly controlled by temperature and the state of the estuary's mouth (i.e. open/closed). Typically, meiofauna abundance in the estuary peaked after periods of prolonged mouth closure and decreased dramatically after the breaching of the estuary at the mouth.

Introduction

Meiofauna constitute a substantial component of marine heterotrophic assemblages (Giere, 1993), influencing the transfer of material and energy through the ecosystem (Li et al., 1997). In estuarine and coastal benthic systems, meiofauna represent an important link between primary producers and higher trophic levels. They are important consumers of microphytobenthos (Blanchard, 1991, Montagna, 1995, Pace and Carman, 1996, Moens et al., 2002, Pinckney et al., 2003) and serve as a food source for other members of the meiobenthos (Elmgren, 1976), benthic macrofauna (Ronn et al., 1988, Walters et al., 1996, Gregg and Fleeger, 1998, Tita et al., 2000) as well as juvenile fish species (Marinelli and Coull, 1987, McCall and Fleeger, 1995).

Although a number of studies have examined the ecology and the role of meiofauna in the energy transfer in South African permanently open estuaries (Dye, 1978a, Dye, 1978b, Dye, 1983a, Dye, 1983b, Dye and Furstenburg, 1978, Dye and Furstenburg, 1981, Dye et al., 1978, De Decker and Bally, 1985, Dye and Lasiak, 1986, Wynberg and Branch, 1994, Heymans and Baird, 1995), nothing is known about temporarily open/closed estuaries in this regard. These systems are closed off from the sea during the dry season, when they receive low or no river inflow and are exposed to the dominant effect of longshore sand movements in the marine nearshore (Day, 1981). During spring/summer, high rainfall and freshwater runoff result in freshwater rising to levels sufficiently high to cause the breaching of the sandbar at the mouth of the estuary. The water level then drops very rapidly, often exposing large areas of substratum, which had been previously submerged and colonized by a rich community of plants and animals (Perissinotto et al., 2000).

The lack of information on the meiofauna of temporarily open/closed estuaries is unfortunate since these systems: (1) represent 73% of South African functioning estuaries (Whitfield, 1998); (2) support a high microphytobenthic biomass (Nozais et al., 2001a, Perissinotto et al., 2002, Mundree et al., 2003); and (3) are important nursery areas for estuarine-dependent fishes (Whitfield and Marais, 1999). Moreover, in estuarine ecosystems, meiofauna are known to play a more significant role than macrobenthos, with respect to benthic energy flow and biogeochemical processes, i.e. remineralization and recycling of organic matter (Castel, 1992).

The purpose of the present investigation was, therefore: (1) to investigate the composition, abundance, and biomass of meiofauna in a typical temporarily open/closed estuary; (2) to estimate the potential grazing impact of meiofauna on the rich microphytobenthic biomass that these estuaries exhibit; and (3) to identify the key variables that affect the meiofaunal community structure and composition on a temporal scale, the emphasis being on the contrast between open and closed phases.