Higher Biovolume Research Articles

Ecological patterns of phytoplankton assemblages in Lake Garda: seasonal, spatial and historical features

The development of phytoplankton assemblages in the two main basins of the deep (Zmax=350 m) and large (49 km3) subalpine Lake Garda (Northern Italy) was investigated through monthly samplings from 1998 to 2000. The algal development was strongly typified by the alternation, from spring to autumn, of Fragilaria crotonensis and Mougeotia sp. and by the increasing importance of filaments of the complex Planktothrix rubescens/agardhii in summer and autumn. These three “master species” are characterised by higher biovolumes and/or more regular and wide annual development in comparison to the other dominant taxa. The simultaneous application of ordination (NMDS-Non metric Multi Dimensional Scaling) and cluster analysis techniques revealed an ordered and coherent temporal succession of phytoplankton assemblages in the two main basins of the lake. These temporal and spatial regularities arise from the high inertia and resilience against perturbations that characterise the deep and large lakes, and contrast sharply with the less ordered or chaotic and unpredictable seasonal assemblages that may be found in small and shallow lakes. It is stressed that, owing to the use of different methodologies, the modifications of the phytoplankton assemblages in the last fifty years must be interpreted with particular caution. The only demonstrable differences seem to indicate that, with the speeding up of eutrophication processes in the 1960s and 1970s, detectable populations of Planktothrix and irregular increases of Conjugatophyceae (Mougeotia sp., Closterium aciculare) were established. During and after the 1980s, the increase of phosphorus concentrations in the lake and the progression towards more mesotrophic conditions was accompanied by the appearance of further substantial populations of cyanobacteria (Planktolyngbya limnetica, Anabaena lemmermannii). In contrast with these signs of alteration, Fragilaria crotonensis, Asterionella formosa, Ceratium hirundinella and Dinobryon spp. were among the most abundant taxa from the 1950s to the present time. These modifications are largely consistent with the changes observed during the 1990s in consequence of the different extent of the deep vertical mixing. During complete overturn, with the maximum spring replenishment of nutrients in the euphotic layers, a greater development of Mougeotia sp. and Oscillatoriales was observed. The historical trend and the ecological results allowed elucidation of the trophic characteristics of many important species developing in Lake Garda and, in general, in the deep subalpine lakes.

Contrasting effects of the invertebrate predator Chaoborus obscuripes and planktivorous fish on plankton communities of a long term biomanipulation experiment

In a long-term biomanipulation-experiment, started in 1979, all planktivorous fish were removed from an experimental lake while a reference lake was left unmanipulated. The intermediate response was characterized by a strong increase in both Daphnia biovolume and size, and an increase in the abundance of the invertebrate predator Chaoborus flavicans. After 12 years, the larger and darker C. obscuripes, typical of fishless lakes, successfully colonized and displaced the smaller species C. flavicans within three years. Subsequently in 1995, zooplankton biovolume and size decreased dramatically, caused by heavy predation of C. obscuripes on all zooplankton from small rotifers to large daphnids. Zooplankton biovolume was substantially lower than in an adjacent reference lake dominated by planktivorous fish. In the reference lake also most of the crustaceans were eliminated but a high biovolume of rotifers remained. Phytoplankton biovolume was considerably higher in the experimental lake than in the reference lake, although no evident differences in primary production and Secchi could be detected between the two lakes. We clearly showed that the total absence of planktivorous fish can lead to worse water quality (in terms of phytoplankton biovolume) than a very high biovolume of planktivorous fish.

Phytoplankton responses to grazing by Daphnia galeata in the biomanipulated Bautzen reservoir

We studied the response of phytoplankton to grazing by Daphnia galeata in the hypertrophic Bautzen reservoir (Dresden, Germany) from January 1995 to May 1996 during a long-term whole-lake biomanipulation experiment. The correlation between clearance rate of D. galeata and total phytoplankton biomass was negative only if biomass of Microcystis aeruginosa was excluded. This suggests that M. aeruginosa is the main grazing resistant phytoplankton species in the Bautzen reservoir. Except for M. aeruginosa and grazing-resistant Staurastrum quadridentatum spec. nov. (Scharf, 1995) no other phytoplankton species was able to reach a biovolume above 1 mm3 L-1 when the clearance rate of Daphnia exceeded 0.1 L L-1 d-1. There was significant positive correlation between mean cell or colony size of phytoplankters and clearance rate of D. galeata, showing an advantage of bigger cells or colonies at high grazing pressure. Cross-correlation indicated a time lag of about one month between changes in grazing pressure and a change in phytoplankton mean size. The phytoplankton species were divided into edible and inedible fractions, depending on their width and length. No edible species were able to reach high biovolumes during high biomasses of D. galeata but a positive correlation was found between the edible fraction of phytoplankton biovolume and the clearance rate of D. galeata. However, this relationship disappeared when the ‘ingestible edible’ fraction of M. aeruginosa was excluded, suggesting a rejection of ‘edible’ Microcystis colonies by D. galeata. A negative correlation was found between the inedible fraction of phytoplankton biovolume without M. aeruginosa and the clearance rate of D. galeata which might be due to superior competition of M. aeruginosa. We could clearly show that biomanipulation might not work well with respect to the reduction of total phytoplankton biomass under hypertrophic conditions and finally discuss a theoretical threshold of phosphorus (probably around 80 μg L-1), below which biomanipulation should become effective.

Response of epilithic microphytobenthos of the Western Baltic Sea to in situ experiments with nutrient enrichment

The response of epilithic microphytobenthos to in situ nutrient enrichments was studied in the Kiel Fjord, Western Baltic Sea. For this purpose an experimental setup with continuous nutrient supply was designed and installed. Experiments followed the colonization of artificial substrates and the responses of benthic algae to different concentrations and combinations of nitrogen and phosphate. They revealed nitrogen limitation of epilithic microflora from late spring to autumn, such that there was higher biovolume with increasing nitrogen concentrations. Diatoms became dominant in all experiments except one in which the rhodophyte Ceramium strictum prevailed. Species composition was altered by nutrient treatments; one to several species were highly favoured by nutrient enrichment. Consequently, diversity was negatively correlated with final yield. These findings support the hypothesis that competition is an important factor structuring microphytobenthic communities.

Life cycle of Cyclops vicinus in relation to food availability, predation, diapause and temperature

The seasonal cycle of Cyclops vicinus was studied during a 5 year period in the shallow and hypertrophic Lake Søbygård. The annual number of generations varied between three and six and sometimes included midsummer generations. Naupliar and copepodite density was extremely high, the maximum recorded being 1313 and 745 1 −1 respectively. The seasonal fluctuation in the abundance of C.vicinus was related to the concentration of edible phytoplankton (EDP): when diatoms and cryptomonads were abundant, nauplii and copepodites were also abundant, whereas when greens such as Scenedesmus spp. were dominant, no cohorts were observed. The maximum density of copepodites in the separate cohorts and the amount of EDP, expressed as carbon content, were found to be significantly correlated. The development of the second cohort also seemed to depend on the occurrence of a peak in rotifer density. High number of eggs per female generally coincided with high biovolume of EDP, suggesting that algae constituted a major part of the diet of adult C. vicinus . The duration of the naupliar stage was inversely related to temperature and is one of the shortest times that have been recorded in the literature. The short duration is probably attributable to the abundance of food. Fish composition and the calculated sex ratio of C.vicinus indicate low predation pressure from planktivorous fish. The high density of C.vicinus during the summer, when the whole population would be expected to have entered diapause, is probably attributable to the concurrence of low predation and the abundance of food.

Patterns of Epipelic Algal Abundance with Depth, Trophic Status, and Acidity in Poorly Buffered New Hampshire Lakes

The biovolume and species composition of algae on sediments in 20 poorly buffered New Hampshire lakes were surveyed in autumn to delineate patterns of community structure and composition with acidity and trophic status of lakes, and along depth gradients within lakes. Patterns of total algal biovolume on sediments were not strongly related to these habitat conditions because of masking by algae that probably settled from the plankton onto sediments. Biovolumes of "benthic" algae generally decreased with depth; however, weak and positive correlations between biovolumes of some benthic algal taxa and depth indicated that benthic algae had adapted to low light conditions and were perhaps facultatively heterotrophic. Decreases in diatom and increases in blue-green algal biovolumes with total phosphorus and chlorophyll a concentrations in lake water indicated that blue-green algae were better adapted for sequestering nutrients in low light environments. The variability in patterns of algal biovolume with acid neutralizing capacity (ANC) and pH indicated that algae had adapted well to different levels of ANC and pH. Biovolumes of blue-greens were negatively correlated with ANC, primarily because of the high biovolumes of various blue-green algae that occurred sporadically in lakes with ANC between 0 and 20 μeg/L and pH between 5.8 and 6.0. Diatom biovolumes generally increased with ANC and pH; however, biovolumes of several species of diatoms, including a Pinnularia, were greatest in low ANC and pH lakes. In general, Eunotia biovolumes decreased and Navicula and Pinnularia biovolumes increased with ANC and pH.