Ostracods as bioindicators to reconstruct past environmental conditions at Lake St Lucia, South Africa.

Abstract

Lake St Lucia, situated along the east coast of South Africa, is a highly dynamic system subject to both tidal and freshwater influences with changing environmental conditions. These fluctuating salinity regimes effect the biological communities within the lake, and thus the resilience of the system. Hypersaline conditions have been recorded within St Lucia as a result of severe and prolonged drought events with mass mortalities ensuing. St Lucia is a highly managed system and has recently received a great deal of attention through monitoring programmes, however, few of these encompass long-term changes spanning more than multi-decadal timescales. This underpins the lack in understanding of natural baseline conditions. As salinity is largely variable and the driving factor behind ecosystem functioning, it remains pertinent to gain a long-term understanding of the system. Ostracods, microscopic bivalved crustaceans, have been noted for their sensitivity environmental factors, salinity in particular. These biological indicators can thus offer important insights into past environmental conditions, yet have been under-utilised in the South African context. Here we present a detailed record of palaeosalinity for the Holocene through fossil ostracod analysis of sediment cores retrieved from Lake St Lucia. A 16 m sediment core was extracted from both North Lake and False Bay, and a 12 m sediment core was extracted from Catalina Bay using a piston corer. Core chronology was based on a total of 29 Accelerator Mass Spectrometry radiocarbon age determinations across the three cores. Preservation potential is greatest in the basal region of the cores, with Catalina Bay yielding the highest species diversity and abundance. Fossil records indicate that North Lake illustrates a dominance in brackish-marine ostracod taxa, whilst False Bay depicts a fresher system overall, due to its sheltered landward positioning, and Catalina Bay has a higher marine influence, likely due to its proximity near the estuary mouth. A transfer function was applied to ostracod data to reconstruct palaeosalinity over the Holocene. Results indicate elevated salinities over the early Holocene (~9500-7250 cal yr BP), and ostracod assemblages vastly differing in richness and abundance from those evident in the late Holocene. The early Holocene saw St Lucia as a deep water system with significant marine influence through various tidal inlets, and hence records reflect a high marine dominance. Ostracods appear to thrive in times of increased marine influence as higher water volumes would have been conducive to continual flow and mixing between basins, facilitating ostracod movement. As Lake St Lucia transitioned from a deep water system to the shallow estuarine system evident today, environmental conditions appear less favourable to ostracods found in North Lake and Catalina Bay. A shallower system is more at risk of desiccation and sediment reworking via wind turbulence, both of which will also hinder fossil success and preservation. Fossil records reflect the transition from marine influenced assemblages to estuarine ostracod assemblages occurred around 7000-6000 cal yr BP, the early to mid Holocene. Today, decreased marine influences are heightened by anthropogenic interventions and modifications, and has consequently put extra pressure on an already stressed ecosystem. Hypersaline events, recently noted in St Lucia literature, are not evident in the ostracod record, likely the results of time averaging through sediment mixing. However, a low species diversity experienced in North Lake and Catalina Bay over the late Holocene is indicative of unstable environmental conditions. As there is significant focus on St Lucia’s management, and in particular its mouth and the relinking to the sea, this study will aid in the decision-making process by providing insight on environmental conditions at a multi-millennial scale.