A conservation palaeobiological approach to assess faunal response of threatened biota under natural and anthropogenic environmental change

Abstract

Abstract. Palaeoecological records are required to test ecological hypotheses necessary for conservation strategies as short-term observations can insufficiently capture natural variability and identify drivers of biotic change. Here, we demonstrate the importance of an integrated conservation palaeobiology approach when making validated decisions for conservation and mitigating action. Our model system is the Razim–Sinoie lake complex (RSL) in the Danube Delta (Black Sea coast, Romania), a dynamic coastal lake system hosting unique Pontocaspian mollusc species that are now severely under threat. The Pontocaspians refer to an endemic species group that evolved in the Black Sea and Caspian Sea basins under reduced salinity settings over the past few million years. The natural, pre-industrial RSL contained a salinity gradient from fresh to mesohaline (18 ppm) until human intervention reduced the inflow of mesohaline Black Sea water into the lake system. We reconstruct the evolution of the RSL over the past 2000 years from integrated sedimentary facies and faunal analyses based on 11 age-dated sediment cores and investigate the response of mollusc species and communities to those past environmental changes. Three species associations (“marine”, “Pontocaspian” and “freshwater”) exist and their spatio-temporal shifts through the system are documented. Variable salinity gradients developed, with marine settings (and faunas) dominating in the southern part of the system and freshwater conditions (and faunas) in the northern and western parts. Pontocaspian species have mostly occurred in the centre of the RSL within the marine–freshwater salinity gradient. Today, freshwater species dominate the entire system, and only a single Pontocaspian species (Monodacna colorata) is found alive. We show that the human-induced reduced marine influence in the system has been a major driver of the decline of the endemic Pontocaspian biota. It urges improved conservation action by re-establishing a salinity gradient in the lake system to preserve these unique species.

Legal Framework for Pontocaspian Biodiversity Conservation in the Danube Delta (Romania and Ukraine)

“…However, covering only the large estuarine habitat complex without further detail, the Bern Convention fails to separate the brackish characteristics of PC habitats from "marine" conditions of the estuarine mouth districts (Gogaladze et al, 2021). This is consequential for PC invertebrate community conservation, since human-induced changes in salinity regime as a result of canal constructions and barrier erections that cause a decline of PC species (Varbanov, 2002;Son, 2007b;Trichkova, 2007;van de Velde et al, 2019), are not formally considered as destruction to the biotope. Indicating salinity regimes in estuarine habitats in ecological management programs is paramount, since all large rivers in the region have a controlled artificial regime of flooding and water use, that negatively affects PC biodiversity (Gogaladze et al, 2021).…”

“…Therefore, representatives of PC invertebrate fauna are unsuitable to act as flagship umbrellas. On the other hand, PC invertebrate species are good bioindicators as they are highly sensitive to dissolved oxygen levels and salinity, water flow, and sedimentation (Zhadin, 1952;van de Velde et al, 2019;Gogaladze et al, 2021). However, they are not yet part of the environmental monitoring and management programs (this work).…”

“…The Danube Delta shelters a unique, aquatic ecological community, known as the Pontocaspian (PC) biodiversity (Popa et al, 2009;van de Velde et al, 2019;Wesselingh et al, 2019), which is characterized by charismatic vertebrate species such as sturgeons, less-known invertebrate groups, such as mollusks and crustaceans, as well as diatoms and dinoflagellates (Grigorovich et al, 2003;Marret et al, 2004). Pontocaspian habitats comprise transitional zones between the freshwater and salt water bodies on coastal plains of the Black Sea and the Sea of Azov, such as lower stretches of rivers, lagoons, delta areas, estuaries, brackish lakes, and bays, as well as the entire Caspian Sea (Zenkevitch, 1963;Gogaladze et al, 2021).…”

“…However, many PC species also inhabit fresh waters in lower reaches of large rivers. Pontocaspian biota is threatened and rapidly declining due to direct anthropogenic drivers, such as damming of rivers, modification of marine and freshwater influx in coastal areas, and invasive species among others (Son, 2007a,b;van de Velde et al, 2019;Gogaladze et al, 2021), as well as indirect drivers, such as limited knowledge on PC species and suboptimal institutional cooperation of stakeholders (Wesselingh et al, 2019;Gogaladze et al, 2020a,b). The legal basis to address the decline of PC biodiversity has not been studied, with the exception of sturgeon species (Reinartz et al, 2012;Munteanu et al, 2013).…”

Large-Scale Geographic Size Variability of Cyprideis torosa (Ostracoda) and Its Taxonomic and Ecologic Implications

“…Since the composition of the ostracode fossils did not change during the considered time a salinity value of today (29 PSU; Mischke et al, 2012) was assumed as reliable estimate of the paleo-salinity. A salinity estimate for the modern samples from the Danube Delta (BS) was taken from the literature (van de Velde et al, 2019). Samples from Lake Bosten (XB) were excluded from size-salinity regression analyses since no salinity data were available.…”