Revisiting Remane’s concept: evidence for high plankton diversity and a protistan species maximum in the horohalinicum of the Baltic Sea

Abstract

Ecological and evolutionary processes have shaped current biodiversity patterns. For brackish-water ecosystems, Remane's Artenminimum ('species minimum') concept argues that taxonomic diversity in organisms is lowest within the horohalinicum, which occurs at salinity 5 to 8. This concept developed from macrozoobenthos data; it originated from, and is still applied to, the geologically young Baltic Sea, the world's largest semi-enclosed, brackish water body with a unique permanent salinity gradient. We re-assessed pelagic biodiversity in the Baltic Sea, which had long remained underestimated. We show that phyto-and zooplankton in Baltic waters exhibit unexpectedly high diversity (> 4000 taxa), with dominance by protists. Protists in the Baltic Sea follow a binomial distribution mode, while metazooplankton diversity decreases exponentially with higher salinity; however, species richness of both groups peaks in the horohalinicum. Drifting within large water masses, planktonic organisms are affected by only moderate salinity fluctuations (compared to benthic species), and thus prosper in brackish environments. The present study challenges Remane's concept for large water bodies with relatively stable salinity gradients and substantiates a novel ecological perspective of the previously overlooked high protistan diversity in brackish waters. We infer that the pronounced adaptability and advanced osmoregulation strategies of protists are the result of large-scale ecological and evolutionary processes. The novel brackish-water biodiversity pattern underpins the proposed protistan species-maximum concept, which refines Remane's model by discriminating between the effects of the horohalinicum on the biodiversity of small motile versus large sessile organisms.

Does plankton diversity peak at intermediate salinities? Comment on Telesh et al. (2011)

“…Conversely, it should be obvious that no estimates can be given for salinities outside the range of observation. However, Telesh et al (2011) extrapolate beyond the salinity range covered by Sagert et al (2008), and indeed predict zero diversity at salinities > 28 (Fig. 2C).…”

“…Based on an analysis of published data, Telesh et al (2011;Mar Ecol Prog Ser 421:1-11) claim that plankton diversity peaks at intermediate salinities in the Baltic Sea. They thereby challenge Remane's curve, which was originally derived from observations of macrozoobenthos (Remane 1934; Zool Anz 7(Suppl):34-74), and conclude that high speciation rates allowed for development of high diversity in microscopic species in the Baltic, as opposed to larger organisms.…”

“…Their analysis results in a hump-shaped diversity pattern for protist plankton along the freshwater-marine continuum, implying maximum diversity at intermediate salinities. Telesh et al (2011) thereby challenge the concept of an 'Artenminimum' (minimum diversity at intermediate salinities) first described for macrozoobenthos in the Baltic Sea by Remane (1934;'Remane's curve'). The analysis by Telesh et al (2011), however, is biased because the authors (1) pool species lists within salinity bands, thereby creating patterns which may be due to methodology, and (2) derive patterns of richness to a large extent from coastal sites which are not representative of the open Baltic Sea.…”

“…The analysis by Telesh et al (2011), however, is biased because the authors (1) pool species lists within salinity bands, thereby creating patterns which may be due to methodology, and (2) derive patterns of richness to a large extent from coastal sites which are not representative of the open Baltic Sea. Moreover, in several cases Telesh et al (2011) do not present actual data, but only smooth lines without confidence intervals, leaving the reader without any information as to how representative these lines might be (their Figs. 2 & 3).…”

“…Hooper et al 2005). Using a number of published datasets on phyto-and zooplankton in the Baltic Sea, Telesh et al (2011) try to approximate patterns of total taxon richness along the salinity gradient by binning taxa within arbitrary salinity bands. Their analysis results in a hump-shaped diversity pattern for protist plankton along the freshwater-marine continuum, implying maximum diversity at intermediate salinities.…”