Pål Brettum

The impact of nutrient state and lake depth on top-down control in the pelagic zone of lakes: A study of 466 lakes from the temperate zone to the Arctic

Using empirical data from 466 temperate to arctic lakes covering a total phosphorus (TP) gradient of 2–1036 gL –1 , we describe how the relative contributions of resource supply, and predator control change along a nutrient gradient. We argue that (a) predator control on large-bodied zooplankton is unimodally related to TP and is highest in the most nutrient-rich and nutrient-poor lakes and generally higher in shallow than deep lakes, (b) the cascading effect of changes in predator control on phytoplankton decreases with increasing TP, and (c) these general patterns occur with significant variations—that is, the predation pressure can be low or high at all nutrient levels. A quantile regression revealed that the median share of the predatorsensitive Daphnia to the total cladoceran biomass was significantly related unimodally to TP, while the 10% and 90% percentiles approached 0 and 100%, respectively, at all TP levels. Moreover, deep lakes (more than 6 m) had a higher percentage of

Diversity predicts stability and resource use efficiency in natural phytoplankton communities.

The relationship between species diversity and ecosystem functioning has been debated for decades, especially in relation to the “macroscopic” realm (higher plants and metazoans). Although there is emerging consensus that diversity enhances productivity and stability in communities of higher organisms; however, we still do not know whether these relationships apply also for communities of unicellular organisms, such as phytoplankton, which contribute ≈50% to the global primary production. We show here that phytoplankton resource use, and thus carbon fixation, is directly linked to the diversity of phytoplankton communities. Datasets from freshwater and brackish habitats show that diversity is the best predictor for resource use efficiency of phytoplankton communities across considerable environmental gradients. Furthermore, we show that the diversity requirement for stable ecosystem functioning scales with the nutrient level (total phosphorus), as evidenced by the opposing effects of diversity (negative) and resource level (positive) on the variability of both resource use and community composition. Our analyses of large-scale observational data are consistent with experimental and model studies demonstrating causal effects of microbial diversity on functional properties at the system level. Our findings point at potential linkages between eutrophication and pollution-mediated loss of phytoplankton diversity. Factors reducing phytoplankton diversity may have direct detrimental effects on the amount and predictability of aquatic primary production.

Regional species pools control community saturation in lake phytoplankton

Recent research has highlighted that positive biodiversity–ecosystem functioning relationships hold for all groups of organisms, including microbes. Yet, we still lack understanding regarding the drivers of microbial diversity, in particular, whether diversity of microbial communities is a matter of local factors, or whether metacommunities are of similar importance to what is known from higher organisms. Here, we explore the driving forces behind spatial variability in lake phytoplankton diversity in Fennoscandia. While phytoplankton biovolume is best predicted by local phosphorus concentrations, phytoplankton diversity (measured as genus richness, G) only showed weak correlations with local concentrations of total phosphorus. By estimating spatial averages of total phosphorus concentrations on various scales from an independent, spatially representative lake survey, we found that close to 70 per cent of the variability in local phytoplankton diversity can be explained by regionally averaged phosphorus concentrations on a scale between 100 and 400 km. Thus, the data strongly indicate the existence of metacommunities on this scale. Furthermore, we show a strong dependency between lake productivity and spatial community turnover. Thus, regional productivity affects beta-diversity by controlling spatial community turnover, resulting in scale-dependent productivity-diversity relationships. As an illustration of the interaction between local and regional processes in shaping microbial diversity, our results offer both empirical support and a plausible mechanism for the existence of common scaling rules in both the macrobial and the microbial worlds. We argue that awareness of regional species pools in phytoplankton and other unicellular organisms may critically improve our understanding of ecosystems and their susceptibility to anthropogenic stressors.

The genus Isoëtes in Scandinavia: An ecological review and perspectives

Abstract Information relating to the Scandinavian quillwort species, Isoetes lacustris L. and I. setacea Lam. (syn. I. echinospora Durieu), is reviewed emphasizing their ecological adaptation to infertile and shaded habitats, where stress results from inadequate levels of light, available carbon and nutrients. In addition, their habitats can feature enhanced disturbance and stress arising from man-made and natural impacts; for example, acidification, eutrophication, water-level alteration, ice-scour and sediment instability. Quillworts have evolved a variety of mechanisms by which these adverse impacts can be mitigated. Such adaptations comprise carbon acquisition through crassulacean acid metabolism (CAM), high root biomass, slow turnover, resource reallocation on the vertical gradient of their habitat and other related features typical for an S-strategist (stress-tolerator). It is emphasized that success of this strategy by no means is guaranteed. In fact, the quillworts are ultimately victims of their own, insufficiently flexible, growth strategy under sustained adverse environmental impacts. Evidently I. setacea fares better than I. lacustris in this respect. This review identified a number of research areas which ought to be further advanced. Specifically we would like to emphasize the need for better insights into the extent and rate of vegetation changes driven by acidification. Contradictory evidence seems to exist on this issue.

Performance of a new phytoplankton composition metric along a eutrophication gradient in Nordic lakes

A new phytoplankton metric is presented, which is developed from a large dataset of Norwegian lakes (>2,000 samples from >400 lakes). In contrast to previous metrics, this index is not built on selected ‘indicative’ taxa, but uses all available taxonomic information at genus and species level. Taxa optima with respect to lake trophic status (derived from total phosphorus concentrations) are used to calculate a phytoplankton trophic index (TI) for each sample. Analysis of the TI shows that phytoplankton communities exhibit highly non-linear responses to eutrophication in Norwegian lakes. Reference lakes are characterized by very similar TIs despite having considerable variation in total phosphorus and chlorophyll a concentrations. TI exhibits a non-linear distribution along the eutrophication gradient which separates unimpacted from impacted sites in the study area. We further show that TI exhibits smaller seasonal variations than chlorophyll a, making it a more reliable indicator for lake monitoring. Implications for its applicability within the WFD are discussed.

Autotroph:herbivore biomass ratios; carbon deficits judged from plankton data

A survey on phytoplankton:zooplankton biomass ratios was performed in 342 Norwegian lakes, covering a wide range in lake size and productivity (total phosphorus: 3–246 μg l−1), but with most localities being oligo- to mesotrophic. Mean phytoplankton biomass was 88 μg C l−1, yet with the majority below 50 μg C l−1and a median of 25 μg C l−1. Total zooplankton biomass displayed a mean and median of 37 and 26 μg C l−1, respectively. Cladocerans were by far the dominant group, making up a median of almost 60% of total zooplankton biomass. Total zooplankton biomass as well as that of major aggregated metazoan taxa (cladocerans, calanoid copepods, cyclopoid copepods and rotifers) all showed a positive, but weak correlation with total phytoplankton biomass. These weak correlations suggest that algal biomass per se is a poor predictor of zooplankton biomass. An average phyto-:zooplankton biomass ratio (C:C) of 2.8 (SD±4.7) was found. 30% of the lakes had a phyto-:zooplankton biomass ratio below unity. While there was no correlation between the phyto-:zooplankton biomass ratio with increasing productivity in terms of P concentration, there was a higher biomass ratio in lakes with high fish predation pressure. The low ratio of phyto-:zooplankton biomass suggest major requirements from non-algal sources of C in the zooplankton diet. The need for dietary subsidizing is also supported by the fact that more than 75% of the lakes had algal biomass less than the estimated threshold for net positive growth of zooplankton, although it should be kept in mind that a high share of picoplankton would imply an underestimation of autotroph biomass in these lakes. Since the C-deficiency apparently is most pronounced in oligotrophic systems, it contradicts the view that the detritus pathways plays a predominant role in highly productive systems only, but while the source of detritus probably is mostly of autochthonous origin in eutrophic lakes, allochthonous detritus will be more important in oligotrophic systems.

Changes in the volume and composition of phytoplankton after experimental acidification of a humic lake

Abstract In connection with the HUMEX project carried out in Lake Skjervatjern, near Forde, in the western part of Norway, the effects of the experimental acidification on the phytoplankton volume and composition were investigated. The lake was divided into two basins by a thick plastic curtain. One of the basins, Basin A, and its entire catchment was acidified by a mixture of ammonium nitrate and sulphuric acid, while Basin B acted as a reference. The results of the phytoplankton analyses from both basins in the investigation period are presented. In Basin B, the control basin, the succession of the main groups of phytoplankton throughout the growth season remained almost identical unit 1993. The results from the acidified Basin A in 1991 and 1992 show marked changes in the phytoplankton composition and percentage of the main algae groups. The percentage of the green algae decreased, especially the species Oocystis submarina v. variabilis , while the dinoflagellates, mainly the species Peridinium umbonatum (P. inconspicuum) , and the cryptomonads increased. In 1993, however, and even more in 1994, there was a marked change in the composition in the control Basin B compared to previous years, with less green algae and more chrysomonads. The total volume was higher in Basin A in 1994 than in Basin B, especially in the autumn, while it was more or less the same in both basins in 1993. In the acidified Basin A, the dominance of the dinoflagellates was more pronounced in 1993 and 1994 than in previous years. The reason for the change in composition of phytoplankton registered in the control Basin B in 1993 and 1994 is hard to explain. As the changes in the composition of Basin A came immediately after the treatment of the Basin started, it is obvious that the addition of nitrogen, and especially the nitrate, through the treatment, was important in regulating the composition in this acid and humic lake. An increase in phosphorus seems to contribute more to the level of the total biomass of phytoplankton than to the observed changes in the composition.

The occurrence and spread of Gonyostomum semen (Ehr.) Diesing (Raphidophyceae) in Norwegian lakes

The invasive and nuisance microalga Gonyostomum semen has recently expanded its geographical distribution and increased its biomass in Sweden and Finland. Temperatures, total organic carbon (TOC), water colour and phosphorous are main factors suggested as drivers for its success. Already in the 1980s there were indications of the same patterns also in Norway, and in this study we further examine these observations based on the hypotheses that (1) G. semen has continued its increase in abundance in Norway since then, (2) after settling in a lake, G. semen may increase its biomass, provided a simultaneous change in climatic factors. We use existing data from Norwegian monitoring databases for the study and additional water samples to investigate distribution. G. semen was detected by microscopy and quantitative PCR, while trends over time in G. semen biomass and possible explanatory variables were analysed by simple regression. We show that G. semen has increased its distribution in Norway since the 1980s, geographically and to new lakes. In eight of the nine lakes studied, the proportion of G. semen biomass in lake phytoplankton has increased with time. These changes seem to be explained only to some degree by temperature, TOC, water colour and phosphorous.

The River Bjerkreim in SW Norway — Successful Chemical and Biological Recovery After Liming

On a large scale, the acidified River Bjerkreim, southwestern Norway, has been treated with lime since the autumn 1996. During the Atlantic salmon (Salmon salar) smolting period pH has been above 6.2 and LAI concentrations below 10 μg L-l. Before 1996, only the western part of the watercourse harboured acid-sensitive species, such as the Atlantic salmon, snails, mayflies, daphnids and Gammarus lacustris. Prior to liming in 1996, Atlantic salmon fry (0+) and parr (≥1+)were found in 4 of 20 sampling sites, contrary to 17 (fry) and 12 (parr) in 1999. Atlantic salmon catches have increased from about 0.8 tons in 1994 to about 10 tons in 1998 and 1999. Acid-sensitive invertebrates have invaded the limed parts of the river.