Palaeolimnological evidence for the independent evolution of neighbouring terminal lakes, the Murray Darling Basin, Australia

Abstract

The estuary of the lower River Murray features a complex mosaic of lakes, coastal lagoons and interconnecting channels. The waters of these wetlands are degraded as a result of river regulation, water abstraction, salinisation, sedimentation and the recent constriction of the River mouth. Palaeolimnologial analysis of sediment cores in two wetlands reveals that salinity in the large terminal Lake Alexandrina was only moderately influenced by tidal inflow, particularly over the past ca. 2000 years. It is now largely fresh as a result of isolation by a series of barriers completed by 1940 AD. In contrast, the seaward portion of the Coorong, a back barrier coastal lagoon, was determined to be a subsaline estuary strongly influenced by marine inflows. These findings contrast somewhat with the Coorong's current Ramsar classification as a saline lagoon. Riverine diatoms, typical of the fossil flora of Lake Alexandrina, are rare or absent in the Holocene sediments of the Coorong, other than for a short period in the late Holocene in the northernmost end of the lagoon. The palaeolimnological evidence for independent evolution of these wetlands is consistent with geomorphic evidence of a stranded, last interglacial shoreline that acted as a sill limiting the exchange of flows between Lake Alexandrina and the Coorong lagoon.

A diatom dataset and diatom-salinity inference model for southeast Australian estuaries and coastal lakes

“…7). This agrees with previous descriptions of Grammatophora marina and Opephora pacifica which have been classified as marine taxa (Vos and de Wolf 1993;Fluin et al 2007). Planothidium hauckianium agg.…”

“…Whilst most of these more abundant taxa have been previously described as 'brackish-marine' taxa, this study permits a finer scale discrimination of their salinity preferences. For example, Cocconeis placentula has previously been identified as a brackish, shallow water species (Hodgson et al 1996;Fluin et al 2007), and this study is consistent with these observations (i.e. it was most abundant in brackish conditions, 5-30 ppt, Fig.…”

“…The most common and widespread taxa (Table 4) are considered cosmopolitan species, characteristic of brackish, coastal and marine water bodies (Witkowski et al 2000) and have been identified in previous Australian coastal diatom studies (e.g. Fluin et al 2007;Haynes et al 2007;Saunders et al 2007;Taffs et al 2008). Table 3 for site codes and water chemistry information This study demonstrated that diatom assemblages in southeast Australian coastal lakes, lagoons and estuaries are primarily influenced by salinity.…”

“…Large-scale environmental problems have arisen due to upstream river regulation, water extraction for irrigation, salinization, enhanced sedimentation rates, modifications of openings to the sea, urbanization of coastal catchments, acidification and eutrophication (SoE 2006;Fluin et al 2007;Taukulis and John 2009). Management efforts are increasingly focused on alleviating environmental problems (SoE 2006).…”

Palaeoecology as a means of auditing wetland condition

“…This lead to calls for the dedication of 700 GL of river water into the system to save the Coorong and adjacent lakes (Gell 2010). Palaeolimnological evidence from cores taken the length of the Coorong revealed it to have a subsaline history with little direct contribution from the river (Fluin et al 2007), as revealed by very low levels of the river plankter Aulacoseira granulata (Figure 3). In fact, it was highly reliant on freshwater contributions from Salt Creek.…”