Does the decline of gastropods in deep water herald ecosystem change in Lakes Malawi and Tanganyika?

Abstract

Summary 1. Ancient, deep lakes have traditionally been considered as stable, ecological islands, well buffered from environmental change because of their great depth. However, they are not immune to anthropogenic and climatic stress. Ecosystems of the permanently stratified warm Lakes Malawi and Tanganyika in the Great East African Rift are particularly delicate. Their stratification regime has historically limited the distribution of benthic biota to a ‘bathtub ring of biodiversity’, namely substrata in the upper, oxygenated water layer. 2. We use historical data on the endemic deep‐water molluscs of these lakes to assess present‐day stress on their benthic ecosystems. During the 20th century, these molluscs have probably decreased in abundance and migrated to shallower water. 3. These apparent trends have a significance beyond species‐based conservation, foremost because deep‐water organisms heavily rely on the position and temporal stability of the oxycline and therefore provide an early warning of large‐scale changes in the distribution of dissolved oxygen. Oxygen demands have increased in the East African Great Lakes over the last century whereas ventilation of deep water has remained the same or declined. 4. The combination of these factors is resulting in a narrowing of the ring of biodiversity and a changed nutrient flux through this ring. Reduction in the habitat available inevitably puts biota at risk, whereas changes in nutrient flux may cause shifts in the entire ecosystem or the collapse of parts of it. 5. Considering the socioeconomic value of these lakes and the potentially grave implications for their faunal biodiversity and entire ecosystems, existing evidence of faunal decline, especially in taxa that depend strongly on the stratification regime, is of great concern. Moreover, because the factors responsible are widespread and include surface‐water warming, increased run‐off and eutrophication, respiration stress may also develop in other tropical and subtropical lakes.

Morphological stasis in an ongoing gastropod radiation from Lake Malawi

“…B. ecclesi currently occurs in our sampling area but is adapted to a habitat that is markedly dissimilar from those occupied by shallow-water species (18). Because this habitat did not exist in the Malawi Basin during the Late Pleistocene megadroughts (19), nor perhaps at the last glacial maximum, it is unlikely that lineage splitting and the divergence toward B. ecclesi started considerably earlier than that in the shallow-water community.…”

“…Four taxonomically valid Bellamya species currently occur in Lake Malawi (16,17): three inhabit shallow water environments (Bellamya capillata, Bellamya jeffreysi, and Bellamya robertsoni), whereas the fourth (Bellamya ecclesi) is rare and occupies substrates below the euphotic zone (16,18). Molecular analysis using three gene fragments [mitochondrial cytochrome oxidase subunit I (COI), the mitochondrial large subunit of rRNA and the nuclear large subunit of rRNA] indicates that all Bellamya from the Malawi Basin form a monophyletic clade endemic to the basin, and although the species are young, great and highly significant genetic differentiation (F ST = 0.234) exists among the three shallow-water species (14).…”

Transparency, Geomorphology and Mixing Regime Explain Variability in Trends in Lake Temperature and Stratification across Northeastern North America (1975–2014)

“…This may ultimately provide a positive feedback for lake warming [11] and result in a longer stratified period with shorter ice cover periods. A longer stratification period each year will exacerbate the extent and duration of hypolimnetic hypoxic or anoxic conditions [21,36,83]. In many lakes, changes in the amount of vertical mixing can also contribute to changes in nutrient dynamics and ultimately lake productivity [84,85] and changing fisheries [86].…”

Climate warming reduces fish production and benthic habitat in Lake Tanganyika, one of the most biodiverse freshwater ecosystems

“…In each case, we quantified geochemical proxies for temperature and algal production as well as the abundance of fossils from pelagic fishes and benthic invertebrates (ostracodes and molluscs). Benthic animals are of special concern because stronger stratification reduces oxygenated habitat in Lake Tanganyika (16,17). In modern sediments, benthic invertebrates are generally absent from sediments deposited under anoxic conditions, although some ostracodes tolerate low oxygen (as low as 1 mg·L ) (17)(18)(19)(20).…”

“…Benthic animals are of special concern because stronger stratification reduces oxygenated habitat in Lake Tanganyika (16,17). In modern sediments, benthic invertebrates are generally absent from sediments deposited under anoxic conditions, although some ostracodes tolerate low oxygen (as low as 1 mg·L ) (17)(18)(19)(20). We quantified trends, correlations, break points in temporal patterns, and cross-factor correlations for temperature, algal production, and fossils to understand the respective roles of lake warming and fishing pressure in the recent history of the remarkable biota of Lake Tanganyika.…”