Testable models of aquatic systems: A NERC special topic for integrating experiments and modelling

A major part of the support for fundamental research on aquatic ecosystems continues to be provided by the Natural Environment Research Council (NERC). Funds are released for ”thematic” studies in a selected special topic or programme. ”Testable Models of Aquatic Ecosystems” was a Special Topic of the NERC, initiated in 1995, the aim of which was to promote ecological modelling by making new links between experimental aquatic biologists and state-of-the-art modellers. The Topic covered both marine and freshwater systems. This paper summarises projects on aspects of the responses of individual organisms to the effects of environmental variability, on the assembly, permanence and resilience of communities, and on aspects of spatial models. The authors conclude that the NERC Special Topic has been highly successful in promoting the development and application of models, most particularly through the interplay between experimental ecologists and formal modellers

Ecophysiology matters: linking inorganic carbon acquisition to ecological preference in four species of microalgae (Chlorophyceae)

The effect of CO2 supply is likely to play an important role in algal ecology. Since inorganic carbon (Ci) acquisition strategies are very diverse among microalgae and Ci availability varies greatly within and among habitats, we hypothesized that Ci acquisition depends on the pH of their preferred natural environment (adaptation) and that the efficiency of Ci uptake is affected by CO2 availability (acclimation). To test this, four species of green algae originating from different habitats were studied. PH-drift and Ci uptake kinetic experiments were used to characterize Ci acquisition strategies and their ability to acclimate to high and low CO2 conditions and high and low pH was evaluated. Results from pH drift experiments revealed that the acidophile and acidotolerant Chlamydomonas species were mainly restricted to CO2, whereas the two neutrophiles were efficient bicarbonate users. CO2 compensation points in low CO2-acclimated cultures ranged between 0.6 and 1.4 µM CO2 and acclimation to different culture pH and CO2 conditions suggested that CO2 concentrating mechanisms were present in most species. High CO2 acclimated cultures adapted rapidly to low CO2 condition during pH-drifts. Ci uptake kinetics at different pH values showed that the affinity for Ci was largely influenced by external pH, being highest under conditions where CO2 dominated the Ci pool. In conclusion, Ci acquisition was highly variable among four species of green algae and linked to growth pH preference, suggesting that there is a connection between Ci acquisition and ecological distribution

The nature of competition between macrophytes and phytoplankton in freshwaters

In field experiments designed to induce dense phytoplankton crops by phosphate and nitrate additions to enclosures in a bed of Potamogeton filiformis in Loch Fitty, the anticipated phytoplankton were not produced. Bioassays showed that phytoplankton were limited by phosphorus and nitrogen. No evidence for an allelopathic effect was found. Macrophyte uptake was responsible for removing 36% of the nitrate added, and the sediment responsible for a part of the phosphate uptake. Some phytoplankton uptake was inferred from the increased zooplankton numbers in enclosures receiving phosphate and nitrate. Nutrient additions had no effect on macrophyte standing crop, as predicted, because the sediment provided an adequate nutrient supply. With decay of macrophytes and nutrient release, phytoplahkton increased in certain enclosures, but not others, probably as a result of large increases in zooplankton numbers and hence grazing pressure. The filamentous alga Rhizoclonium became abundant at the end of the season in enclosures receiving phosphate and nitrate, but did not appear to harm the macrophytes. Epiphytes were only visibly obvious in one enclosure. Failure to produce dense phytoplankton crops in the field led to a laboratory study of the effects of phytoplankton-induced carbon competition on macrophytes. Phytoplankton species were shown to have a smaller total resistance to CO2 fixation than macrophytes and hence greater photosynthetic rates under most CO2 concentrations. The boundary layer was the largest component of the total resistance in macrophytes, suggesting that the thin leaves of many macrophytes were a response to this rather than an aid to diffusion. The linear leaves of other species could be adaptations to reduce the boundary layer thickness. A pH-drift technique confirmed that the best phytoplankton species were more efficient at carbon removal than any macrophyte shoots. The macrophytes were even less efficient when the whole plant was considered. The carbon compensation point was shown to rise under the low light conditions that would be found under a dense phytoplankton crop. Macrophytes showed seasonal changes in carbon extractive ability, but the range was less than published data for phytoplankton from a lake, probably because the latter consists of a series of populations, which are closely adapted to the prevailing conditions. Different leaf types of heterophyllous macrophytes had different CO2 compensation points and one leaf type could use HCO3. A growth experiment confirmed that carbon competition with phytoplankton could have a detrimental effect on macrophytes

Insights on the functions and ecophysiological relevance of the diverse carbonic anhydrases in microalgae

Carbonic anhydrases (CAs) exist in all kingdoms of life. They are metalloenzymes, often containing zinc, that catalyze the interconversion of bicarbonate and carbon dioxide—a ubiquitous reaction involved in a variety of cellular processes. So far, eight classes of apparently evolutionary unrelated CAs that are present in a large diversity of living organisms have been described. In this review, we focus on the diversity of CAs and their roles in photosynthetic microalgae. We describe their essential role in carbon dioxide-concentrating mechanisms and photosynthesis, their regulation, as well as their less studied roles in non-photosynthetic processes. We also discuss the presence in some microalgae, especially diatoms, of cambialistic CAs (i.e., CAs that can replace Zn by Co, Cd, or Fe) and, more recently, a CA that uses Mn as a metal cofactor, with potential ecological relevance in aquatic environments where trace metal concentrations are low. There has been a recent explosion of knowledge about this well-known enzyme with exciting future opportunities to answer outstanding questions using a range of different approaches

Regulation of carbon metabolism by environmental conditions: a perspective from diatoms and other chromalveolates

Diatoms belong to a major, diverse and species-rich eukaryotic clade, the Heterokonta, within the polyphyletic chromalveolates. They evolved as a result of secondary endosymbiosis with one or more Plantae ancestors, but their precise evolutionary history is enigmatic. Nevertheless, this has conferred them with unique structural and biochemical properties that have allowed them to flourish in a wide range of different environments and cope with highly variable conditions. We review the effect of pH, light and dark, and CO2 concentration on the regulation of carbon uptake and assimilation. We discuss the regulation of the Calvin-Benson-Bassham cycle, glycolysis, lipid synthesis, and carbohydrate synthesis at the level of gene transcripts (transcriptomics), proteins (proteomics) and enzyme activity. In contrast to Viridiplantae where redox regulation of metabolic enzymes is important, it appears to be less common in diatoms, based on the current evidence, but regulation at the transcriptional level seems to be widespread. The role of post-translational modifications such as phosphorylation, glutathionylation, etc., and of protein-protein interactions, has been overlooked and should be investigated further. Diatoms and other chromalveolates are understudied compared to the Viridiplantae, especially given their ecological importance, but we believe that the ever-growing number of sequenced genomes combined with proteomics, metabolomics, enzyme measurements, and the application of novel techniques will provide a better understanding of how this important group of algae maintain their productivity under changing conditions

When phenology matters: age–size truncation alters population response to trophic mismatch

Climate-induced shifts in the timing of life-history events are a worldwide phenomenon, and these shifts can de-synchronize species interactions such as predator–prey relationships. In order to understand the ecological implications of altered seasonality, we need to consider how shifts in phenology interact with other agents of environmental change such as exploitation and disease spread, which commonly act to erode the demographic structure of wild populations. Using long-term observational data on the phenology and dynamics of a model predator–prey system (fish and zooplankton in Windermere, UK), we show that age–size truncation of the predator population alters the consequences of phenological mismatch for offspring survival and population abundance. Specifically, age–size truncation reduces intraspecific density regulation due to competition and cannibalism, and thereby amplifies the population sensitivity to climate-induced predator–prey asynchrony, which increases variability in predator abundance. High population variability poses major ecological and economic challenges as it can diminish sustainable harvest rates and increase the risk of population collapse. Our results stress the importance of maintaining within-population age–size diversity in order to buffer populations against phenological asynchrony, and highlight the need to consider interactive effects of environmental impacts if we are to understand and project complex ecological outcomes

Ecological imperatives for aquatic CO2-concentrating mechanisms

In aquatic environments, the concentration of inorganic carbon is spatially and temporally variable and CO2 can be substantially over-saturated or depleted. Depletion of CO2 plus low rates of diffusion cause inorganic carbon to be more limiting in aquatic than terrestrial environments and the frequency of species with a CCM, and their contribution to productivity is correspondingly greater. Aquatic photoautotrophs may have biochemical or biophysical CCMs and exploit CO2 from the sediment or the atmosphere. Though partly constrained by phylogeny, CCM activity is related to environmental conditions. CCMs are absent or down-regulated when their increased energy costs, lower CO2 affinity or altered mineral requirements outweigh their benefits. Aquatic CCMs are most widespread in environments with low CO2, high HCO3-, high pH and high light. Freshwater species are generally less effective at inorganic carbon removal than marine species but have a greater range of ability to remove carbon, matching the environmental variability in carbon availability. The diversity of CCMs in seagrasses and marine phytoplankton and detailed mechanistic studies on larger aquatic photoautotrophs are understudied. Strengthening the links between ecology and CCMs will increase our understanding of the mechanisms underlying ecological success and will place mechanistic studies in a clearer ecological context

The ecology of Bassenthwaite Lake (English Lake District)

Bassenthwaite Lake is, in many ways, different from the other major lakes in the English Lake District: it is the most northerly, the shallowest, has the largest catchment and the shortest mean retention time. There is also considerable temporal variation in lake level. Although the lake is surrounded by rich lowland soils, the catchment is dominated by upland moor and improved pasture, underlain by four dominant soil types: well drained loam with bare rocks and scree, shallow acid upland peat, fine loam and thick acidic peat soils. The supply of catchment-derived sediment, and its recycling within the lake, has serious implications for water quality and for the ecology of this site. These sediments largely comprise inorganic material, and are believed to originate mainly from the high fells and the floodplain of the River Derwent.Typically the lake thermally stratifies between May and September, and is mixed for the remainder of the year. However, the lake is only ever weakly stratified and episodic mixing events influence the ecology of the lake, by increasing the concentration of oxygen at depth and resuspending sediment in shallow water.Palaeolimnological reconstruction of long-term changes in total phosphorus concentration has revealed that, over the past 250 years, the lake has become increasingly eutrophic, with increases in phosphorus concentration being particularly dramatic in the late 1970s and early 1980s. Though tertiary sewage treatment was implemented at Keswick (the principal town in the catchment) in 1995, there has been only a slight reduction in phosphorus concentration in the lake. At present, diffuse sources contribute more to the total phosphorus load than do point sources and it is suspected that efficient internal recycling of a relatively modest sediment store, caused by wind-driven resuspension, may be one reason for the limited reduction in phosphorus availability

Dissipation and mixing during the onset of stratification in a temperate lake, Windermere

Acoustic Doppler Current Profilers and chains of temperature sensors were used to observe the spring transition to stable stratification over a 55 day period in a temperate lake. Observations of the flow structure were complemented by measurements of dissipation, based on the Structure Function method, near the lake bed and in the upper part of the water column. During complete vertical mixing, wind-driven motions had horizontally isotropic velocities with roughly equal barotropic and baroclinic kinetic energy. Dissipation was closely correlated with the wind-speed cubed, indicating law of the wall scaling, and had peak values of ~1 x 10-5.5 W kg-1 at 10 m depth during maximum wind forcing (W~ 15 m s-1). As stratification developed, the flow evolved into a predominantly baroclinic regime dominated by the first mode internal seiche, with root mean square (rms) axial flow speeds of ~2-3 cm-1; ~ 2.5-times the transverse component. At 2.8 m above the bed, most of the dissipation occurred in a number of strong maxima coinciding with peaks of near-bed flow. In the pycnocline, dissipation was low most of the time, but with pronounced maxima (reaching ~1 x 10-5 W kg-1) closely related to the local velocity shear. The downward diffusive heat flux across the pycnocline over 27.5 days accounted for ~ 70% of the temperature rise in the water column below. Total lake kinetic energy increased by a factor of 3 between mixed and stratified regimes, in spite of reduced wind forcing, indicating less efficient damping in stable conditions