Thomas Hübener

Downstream changes in phytoplankton composition and biomass in a lowland river–lake system (Warnow River, Germany)

To determine longitudinal changes in phytoplankton composition and biomass in the Warnow River (Germany), single water parcels were followed during their downstream transport in August and October 1996 and April 1997. In summer, the phytoplankton assemblage was dominated by centric diatom and cyanobacteria species. Stephanodiscus hantzschii, Pseudanabaena limnetica, Planktothrix agardhii and Aulacoseira granulata var. angustissima were the most frequent species. In autumn, small centric diatoms dominated the whole river course. Irrespective of the season, in the fluvial lakes of the upper river, a substantial increase of phytoplankton biomass was observed. Shallow upstream river stretches were associated with large biomass losses. In the deep, slow flowing lower regions, total biomass remained constant. Longitudinal changes in biomass reflected downstream variations in flow velocity and river morphology. Cyanobacteria, cryptophytes and diatom species were subjected to large biomass losses along fast flowing, shallow river sections, whereas chlorophytes were favoured. Diatoms and cryptophytes benefited from low flow velocity and increased water depth in the downstream river. Changes in water depth and flow velocity have been found as key factors that cause the longitudinal differences in phytoplankton composition and biomass in small rivers.

Dissolved and particulate phosphorus forms in a eutrophic shallow lake

Abstract. Soluble and particulate phosphorus forms in the surface water of the eutrophic shallow Lake Bützow were investigated from March to November 1998. Soluble reactive phosphorus (SRP) and dissolved organic phosphorus (DOP) of the filtered water were analysed. Suspended particulate matter (SPM) was characterized by particulate organic matter (POM), particulate organic carbon (POC), particulate phosphorus (PP), particulate iron (PFe), phytoplankton biomass (PB) and Chl a. PP was investigated in more detail by means of sequential chemical extraction (Psenner et al., 1984) and the analysis of polyphosphates and phospholipids.¶ In spring, the lowest SRP values and highest PP values were recorded. Over the course of the year, SRP did not decline, whereas DOP increased and became the dominant P-pool in the lake in autumn. Polyphosphate, as a reserve compound in algae, was present only until July. Its decline was accompanied by an increase in phospholipids and a decrease in the easily available PP and the sorptive-bound PP. The levels of iron-bound phosphorus and the apatite-phosphorus remained stable over the year. The available phosphorus declined, although the SRP level never dropped below 0.015 mg L-1. Thus, insufficient phosphorus was available in this eutrophic lake to allow formation of polyphosphate granules in algae in the second half of the year. Neither SRP nor the elemental composition of seston is suitable to describe the P-status and deficiency for algal growth in the shallow Lake Bützow.

Vertical gradient of nutrients in two dimictic lakes - influence of phototrophic sulfur bacteria on nutrient balance

Abstract.Vertical profiles of soluble and particulate nutrients were analyzed at the end of summer stratification in two dimictic lakes located in northeast Germany. In addition, irradiance and plankton biomass were determined. The concentrations of particulate organic carbon and phytoplankton biomass in the epilimnion were higher in Lake Tiefer than in Lake Dudinghausen, even though the apparent trophic status of Lake Tiefer was higher than Lake Dudinghausen. In Lake Dudinghausen, phototrophic sulfur bacteria accumulated in the hypolimnion between 8 and 10 m, whereas in Lake Tiefer low light penetration prevented the development of phototrophic bacteria in those horizons in which sulfide might be present. Because both lakes have anoxic hypolimnia, we assumed that in both cases phosphorus was released from the sediment into the hypolimnion. In Lake Tiefer, redox conditions and the presence of nitrate and nitrite limited the water depth range in which P-release occurred. In Lake Dudinghausen, part of the released soluble reactive phosphorus was incorporated into the phototrophic sulfur bacteria biomass and thus transformed to particulate phosphorus. As much as 70% of the particulate phosphorus in the hypolimnion was found in the phototrophic sulfur bacterial layer, with 15–20% of this particulate phosphorus consisting of polyphosphate storage compounds. The low ratio of soluble reactive phosphorus to particulate phosphorus in the hypolimnion was, therefore, attributed to phototrophic sulfur bacteria. The phototrophic sulfur bacteria appear to act as an internal nutrient filter and convert soluble reactive phosphorus into particulate phosphorus.

A comparison of relative abundance versus class data in diatom-based quantitative reconstructions

Relative species abundances are the most frequently applied data type used for modern or paleolimnological diatom studies. In contrast, plant ecologists save time by commonly using ordinal scale data (class data), where the abundance of a species is estimated using dominance classes, instead of relative abundance data. This study compares the performance of models based on ordinal diatom species class data (class 1: sporadic (<0-1%) up to class 6: dominant (>60%)) with similar model types based on relative abundance data for different regional training sets and sediment cores. First, relative diatom abundances were converted into ordinal classes. Species response to total phosphorous (TP) was modelled using both types of data - relative abundance and ordinal class data. Secondly, TP was reconstructed for six sediment cores from North-East Germany, Switzerland, and Denmark using WA and WA-PLS based on both types of data. Thirdly, 20 lake sediment surface samples with known relative diatom abundances and known water TP concentrations were recounted using an ordinal data scale to create an independent test set. No significant differences were found between relative abundance and class data for (1) explained species variance, (2) reconstructed TP values, and (3) inferred TP values of the 20 recounted samples. This approach demonstrates that past TP concentrations may also be reliably reconstructed using class data instead of relative diatom abundances. Thus, by using class data lake managers may not only obtain more long-term records past water quality, but this approach is also quicker and therefore more cost effective. Moreover, the findings of this study may also advance the use of automatic diatom identification with digital image recognition, as we demonstrate that not every damaged diatom valve needs to be identified.

Food algae for Lesser Flamingos: a stocktaking

Lesser Flamingo, the flagship species of saline wetlands of Africa and India, is a specialised feeder subsisting on microscopic cyanobacteria and algae. To establish the relationship between flamingo occurrence and food algal abundance and quality, an extensive microphyte survey in more than 150 sampling trips to seven countries over a 15-years period (2001–2015) was carried out. The 44 habitat sites included the core soda lakes in eastern Africa (Bogoria, Nakuru, Elmentaita, Oloidien), where the highest numbers of flamingos were observed, and five breeding sites in eastern and southern Africa as well as in north-western India. A reference describing the diversity of microphytes was established including members of three orders of cyanobacteria and nine orders of eukaryotic algae that potentially could act as food source for Lesser Flamingos. Preferred food organisms consisted of filamentous cyanobacteria, mainly Arthrospira, as well as benthic diatoms. Further investigation on the suitability of other microphytes as alternative flamingo diet revealed the food potential of chlorophytes and euglenophytes. This paper discusses a phycological perspective in the feeding ecology of Lesser Flamingos. The survey findings can assist scientists and conservationists in evaluating the potential of wetlands to support flocks of this endangered bird.

An investigation into the morphology and genetics of Cyclotella comensis and closely related taxa

The planktonic freshwater diatom Cyclotella comensis Grunow is widely used as an indicator of oligotrophic lake conditions. Taxonomic confusion arose with the discrimination of the morphologically similar species Cyclotella pseudocomensis Scheffler and Cyclotella costei Druart & Straub. In this study, we used morphological and molecular data to investigate the relationship between C. comensis, C. pseudocomensis and C. costei using 11 strains from eight lakes in Germany and Austria. Morphological variability was high among the three morphospecies such that they could not be separated unambiguously using morphological traits. DNA sequence analysis revealed only minor differences in 18S V4 (<0.25%), 5.8S rDNA (all sequences identical), LSU D2/D3 (<0.8%), rbcL (<0.4%) and cox1 (<0.79%) between the cultures. Cyclotella comensis, C. pseudocomensis and C. costei are indistinguishable on morphological and DNA sequence data, suggesting that they might comprise one taxon. However, it cannot be completely excluded that they are recently diverged, closely related species.

Hazards for the Rostock water supply from a tar-contaminated sediment in the river Warnow

By the gas plant at Schwaan (a small city near Rostock), tar and other by-products were released into the river Warnow which supplies Rostock with drinking water 20 km downstream. The tar-contaminated river sediment contained phenols and polycyclic aromatic hydrocarbons (PAH), indicating a mutagenic potential. Investigations in the catchment area and on tributaries showed PAH contents up to about 10 ng/l in the water. The river sediment was found to be the sole PAH source. Although the PAH level is very low, sediment disturbances may produce an increase in concentrations. The consequences of such a worst case are difficult to estimate, even though the content of seston and humic substances in the river seems to reduce the PAH concentrations. Although this lowers the probability that increased PAH concentrations reach the Rostock water supply, the removal of the tar-contaminated sediment from the river is strongly recommended.