Peter Blomqvist

ALLOCHTHONOUS ORGANIC CARBON AND PHYTOPLANKTON/BACTERIOPLANKTON PRODUCTION RELATIONSHIPS IN LAKES

Humic lakes with high inputs of allochthonous dissolved organic carbon have a pelagic food chain that, to a large extent, is based on bacterioplankton energy mobilization from allochthonous organic carbon compounds. This is in contrast to clear lakes in which total pelagic production is based mainly on phytoplankton photosynthesis. The energy economy in humic lakes may be less efficient than in clear lakes, because it is likely that one more link is included in the food chain. Lake data from Scandinavia and North America demonstrate that shifts between food chains based on heterotrophic production and food chains based on primary production can take place at moderate increases or decreases in the concentration of dissolved organic carbon from allochthonous sources. Large variations in the loading of allochthonous organic carbon (e.g., due to climatic variations) may have considerable effects on the biostructure and productivity of lakes.

Effects of Additions of DOC on Pelagic Biota in a Clearwater System: Results from a Whole Lake Experiment in Northern Sweden

An oligotrophic clearwater lake, initially characterized by a pronounced dominance of autotrophic phytoplankton and mostly by one species, the green alga Botryococcus, was subject to additions of dissolved organic carbon in the form of white sugar (sucrose) during two consecutive years. The hypothesis tested was that it is organic carbon per se, and not other possible effects of humic substances, that determines the differences in structure of the planktonic ecosystem between humic and clearwater lakes. The additions of DOC resulted in a significant increase in bacterial biomass and a decrease in the biomass of autotrophic phytoplankton. The biomass of mixotrophic and heterotrophic flagellates instead increased significantly, whereas no effects were found to propagate to higher trophic levels. As a result of the changes among biota, total planktonic biomass also decreased to a level typical of nearby humic lakes. We suggest that it is the carbon component of humic material and its utilization by bacterioplankton that determines the structure and function of the pelagic food web in humic lakes.

Recruitment of Microcystis (Cyanophyceae) from lake sediments: The importance of littoral inocula

Recruitment of Microcystis from sediments to the water column was investigated in shallow (1–2 m) and deep (6–7 m) areas of Lake Limmaren, central Sweden. Recruitment traps attached to the bottom were sampled on a weekly basis throughout the summer season (  June–September). A comparison between the two sites showed that the recruitment from the shallow bay was significantly higher over the entire season for all three Microcystis species present in the lake. Maximum rates of recruitment were found in August, when 2.3 × 105 colonies m−2·day− 1 left the sediments of the shallow area. Calculated over the entire summer, Microcystis colonies corresponding to 50% of the initial abundance in the surface sediments were recruited in the shallow bay, whereas recruitment from the deep area was only 8% of the sediment colonies. From these results we conclude that shallow areas, which to a large extent have been overlooked in studies of recruitment of phytoplankton, may be crucial to the dynamics of these organisms by playing an important role as inoculation sites for pelagic populations.

Bacterial grazing by phagotrophic phytoflagellates in a deep humic lake in northern Sweden

Bacterial grazing was measured from June to August 1995 in Lake Ortrasket, a deep brown-water lake in northern Sweden. Mixotrophic chrysophytes were the dominating bacterivores at all times, grazin ...

Competition between the green alga Scenedesmus and the cyanobacterium Synechococcus under different modes of inorganic nitrogen supply

In this study, we evaluated growth responses of the green alga Scenedesmus and the cyanobacterium Synechococcus supplied with inorganic nitrogen in different ways. A competitive situation in which nitrogen was limiting was created in mixed cultures as well as in cultures growing in the same vessel but separated by a permeable dialysis membrane. Supplying inorganic nitrogen in small pulses at a high frequency favoured the cyanobacterium Synechococcus, whereas batch additions favoured the green alga Scenedesmus. When using a large-pulse/low-frequency supply mode, the yield of the green alga was higher when ammonium was added as nitrogen source compared to when nitrate was added. By contrast, the yield of the cyanobacterium was higher in the nitrate regime. However, uptake experiments using unialgal cultures showed that both organisms depleted the medium of ammonium more rapidly than they depleted the medium of nitrate; i.e. the higher yield of the cyanobacterium in the nitrate regime than in the ammonium regime can be attributed to the effects of competition with the green alga. Since nitrate assimilation involves the consumption of reductive power, we suggest that the outcome of competition was governed by the fact that green alga was light limited and therefore better able to compete for ammonium than for nitrate. The results from the laboratory studies are discussed in relation to results from an enclosure experiment performed in Lake Erken, Sweden. In that field experiment, in which additions of both phosphate and ammonium were applied every second day to 350-l enclosures, the green algal biomass increased exponentially during an incubation period of 22 days.

An experimental and palaeoecological study of algal responses to lake acidification and liming in three central Swedish lakes

Contemporary phytoplankton and palaeolimnological studies were made of the algal response to acidification and liming in three lakes in Halsingland, central Sweden (Njupfatet, Sjosjon, Djuptjarn). Surveys and experimental studies of the phytoplankton response to liming were undertaken at Njupfatet, together with an experiment designed to determine the possible role of lake sediments as an inoculum for any new species arriving in the water column. Liming had little quantitative or qualitative effect on the phytoplankton diversity at Njupfatet, but did result in the loss of the dominant contributor to algal biomass, Merismopedia tenuissima. None of the four new species recorded in the lake following liming was hatched from the sediment inoculum experiment. A freeze core from Njupfatet was dated by 210PPb and carbonaceous fly-ash particle profiles were also determined for Njupfatet and Sjosjon, which provided an approximate chronology for the latter lake. Diatom analyses were made of the three lakes and pH a...

Production and food web interactions of Arctic freshwater plankton and responses to increased DOC

A gradient of dissolved organic carbon (DOC) was added to enclosures in a high Arctic lake (Svalbard, 79° N). The aim was to simulate the effect of increased concentrations of DOC that will be a predicted effect of increased temperature and precipitation. The study aimed to provide information on the overall effects of such increased levels of DOC on the pelagic food-web, as well as the increased attenuation of UV-radiation (UV-R) caused by increased DOC. The biomass development of phytoplankton, heterotrophic bacteria and ciliates from 15. July (shortly after ice-off) to 8. August revealed a consistent pattern across all enclosures. Initial phytoplankton biomass decreased from maxima around 600μg Cl -1 towards 50μg Cl -1 by the end of July. Similarly, ciliate biomass decreased from ∼100 to 5 μg Cl -1 , while heterotrophic bacteria decreased from initially 280μg Cl -1 to biomasses near 100μg Cl -1 . Over the same period, zooplankton biomass (almost a monoculture of Daphnia tenebrosa) increased from <40 to some 170μg Cl -1 . These patterns were reflected also in decreasing absolute production of bacteria and phytoplankton, while production:biomass ratio remained fairly constant. In general, the study demonstrated a very dynamic system over the brief ice-free season, where Daphnia grazing had a strong impact on the unicellular biota. Additions of DOC initially stimulated planktonic production, yet this effect was soon overruled by Daphnia grazing. This stimulating effect could be a result both of increased UV-R attenuation and some growth stimulating effect. Since no corresponding stimulating effect was observed in the bag shielded with Mylar filter to screen off UV-B, the latter cause seems most likely.

The influence of water colour and effective light climate on mixotrophic phytoflagellates in three small Swedish dystrophic lakes

The influence of water colour and effective light climate on mixotrophic phytoflagellates in three small Swedish dystrophic lakes

A proposed standard method for composite sampling of water chemistry and plankton in small lakes

In this paper a method for collection of vertically and horizontally integrated volume-weighted composite samples for analysis of water chemistry and plankton is presented. The method, which requires a proper knowledge of lake morphometry parameters, includes proposed standard procedures for determination of sampling interval thickness, maximum depth of sampling, selection of sampling stations, and distribution of discrete samples. An example of the outcome of the method in a lake with uncomplicated basin morphometry is given and the results are discussed against background of general lake basin morphometry data. The aim of the paper is to start a debate about optimization (statistical as well as ecological) of volume weighted composite sampling.

Phytoplankton responses to biomanipulated grazing pressure and nutrient additions — enclosure studies in unlimed and limed Lake Njupfatet, central Sweden

Enclosure experiments aimed to assess the role of Chaoborus larvae in regulating lower trophic levels under natural and nutrient-enriched conditions were performed in situ in the dimictic, oligotrophic clearwater Lake Njupfatet, central Sweden. One experiment was performed before, and one after, whole-lake calcite treatment. In the acidic system, total planktonic biomass (TPB; the sum of phyto-, bacterio-, protozoo-, and metazooplankton), dominated by the cyanobacterium Merismopedia, was only weakly stimulated by the nutrients while the predator had no effect. In the limed environment, TPB increased strongly in response to the nutrients in combination with a negative response to the predator. Principally phytoplankton, but also bacterioplankton and protozooplankton (ciliates) were stimulated by the nutrients. It is argued that Merismopedia acted as a dead-end in the energy flow in the acidic system. Liming resulted in the disappearance of Merismopedia and this opened the flow of material to higher trophic levels. Since liming also led to a significant reduction in the concentrations of phosphorus in the lake water, it is proposed that a combination of liming and gentle fertilization might be used to restore the productivity of acidified lakes.