Modelling the effects on phytoplankton communities of changing mixed depth and background extinction coefficient on three contrasting lakes in the English Lake District

1. The process‐based phytoplankton community model, PROTECH, was used to model the response of algal biomass to a range of mixed layer depths and extinction coefficients for three contrasting lakes: Blelham Tarn (eutrophic), Bassenthwaite Lake (mesotrophic) and Ullswater (oligotrophic). 2. As expected, in most cases biomass and diversity decreased with decreasing light availability caused by increasing the mixed depth and background extinction coefficient. The communities were generally dominated by phytoplankton tolerant of low light. Further, more novel, factors were identified, however. 3. In Blelham Tarn in the second half of the year, biomass and diversity did not generally decline with deeper mixing and the community was dominated by nitrogen‐fixing phytoplankton because that nutrient was limiting to growth. 4. In Bassenthwaite Lake, changing mixed depth influenced the retention time so that, as the mixed depth declined, the flushing rate in the mixed layer increased to the point that only fast‐growing phytoplankton could dominate. 5. In the oligotrophic Ullswater, changing the mixed depth had a greater effect through nutrient supply rather than light availability. This effect was observed when the mixed layer was relatively shallow (<5.5 m) and the driver for this was that the inflowing nutrients were added to a smaller volume of water, thus increasing nutrient concentrations and algal growth. 6. Therefore, whilst changes in mixed depth generally affect the phytoplankton via commonly recognized factors (light availability, sedimentation rate), it also affected phytoplankton growth and community composition through other important factors such as retention time and nutrient supply.     

Inter- and intra-specific differences in the number of larval instars in British populations of 24 species of stoneflies (Plecoptera)

1. Ontogenetic changes during the life cycle of aquatic insects are important not only in life‐history studies but also in evaluating food‐web structure. They require information on the growth and number of larval instars but such information is lacking for many species, including Plecoptera. Therefore, the chief objectives of the present study were to determine inter‐ and intra‐specific differences in the number of larval instars in British populations of 24 species of stoneflies, to test Dyar’s hypothesis that growth followed a geometric progression, and to synthesise this information with previously published values for four British species. 2. Larvae were reared at constant temperatures in the laboratory from eggs from 63 populations (one to six populations per species). First instars from each population were divided into three batches and each batch was reared at one of three constant temperatures. For each species, the rearing temperature and source population had no significant effect on the mean size of each larval instar. 3. The relationship between the geometric mean length of each instar and instar number was well described by an exponential equation (P < 0.001, r² > 0.9 for all species), thus supporting Dyar’s hypothesis. Only one species, Brachyptera risi, had the same number of instars for males and females (12–13). For the other 15 herbivorous species and the four smaller carnivorous ones, the number of instars was higher for females than males (range 11–16 for males, 12–17 for females). The larger size of the females was due to their additional instars, not a sex difference in growth rates. In contrast, there was a clear growth separation of the sexes after the 9th or 10th instar for the four largest carnivores. The number of larval instars was highest for these four species (range 16–19 for males, 18–23 for females), and females were much larger than males. 4. A multiple regression equation with data from the present and previous studies (n = 27) showed that variability in the mean length of the first instar and the maximum number of larval instars for each species accounted for 88% and 91% of the variability in the mean length of the final instar for males and females, respectively. 5. Values for Plecoptera in other countries were in general agreement with those in the present study, especially in the same families. Two old, but widely quoted, high values are doubtful. The present study and four previous ones provide a sound basis for ontogenetic studies on 28 species of Plecoptera and their role in aquatic ecosystems.     

Critical periods in the life cycle and the effects of a severe spate vary markedly between four species of elmid beetles in a small stream

1. The chief objectives were: (i) to describe quantitatively the life cycles of four species of Elmidae, Elmis aenea, Esolus parallelepipedus, Oulimnius tuberculatus and Limnius volkmari; (ii) to use life tables to identify critical periods for survival in the life cycle of each species; (iii) to evaluate the immediate and longer‐term effects of a severe spate on densities of the four species. Monthly samples were taken over 63 months at two contrasting sites in a small stream: one in a deep section with macrophytes abundant, and the other in a shallow stony section. 2. There were five larval instars for O. tuberculatus, seven for L. volkmari and six for the other two species. The life cycle of each species took 1 year from egg hatching (chiefly in June for E. aenea and O. tuberculatus, and July for the other species) to pupation in the stream bank and a further year before the adults in the stream matured and laid their eggs. Mature adults were present in most months, but were rare or absent in January and February and attained maximum densities in April for O. tuberculatus and May for the other species. 3. Laboratory experiments provided data on egg hatching and pupation periods and the number of eggs laid per female. Life tables compared maximum numbers per square metre for key life‐stages. Within each species, mortality rates between adjacent life‐stages were fairly constant among six cohorts and between sites, in spite of large differences in numbers. The only exception for all species was the high adult, but not larval, mortality during a severe spate. 4. Standardised life tables, starting with 1000 eggs, identified key life‐stages with the highest mortality, namely the early life‐stages for E. aenea (36% mortality), start of the overwintering period to pupation for O. tuberculatus (41%) and L. volkmari (51%), start of pupation to the maximum number of immature adults for E. parallelepipedus (41%) and between the maximum numbers of immature and mature adults for O. tuberculatus (41%). Therefore, critical periods for survival in the life cycle differed between species, presumably because of their different ecological requirements. Similarly, the effects of the spate on adult mortality, and hence egg production, varied between species, being most severe and long‐term for E. aenea and O. tuberculatus, less severe for E. parallelepipedus and least severe with a rapid recovery for L. volkmari. Possible reasons for these discrepancies are discussed, but more data are required on the food and microhabitat requirements of the elmids before satisfactory explanations can be found.     

The seasonal sensitivity of Cyanobacteria and other phytoplankton to changes in flushing rate and water temperature

The phytoplankton lake community model PROTECH (Phytoplankton RespOnses To Environmental CHange) was applied to the eutrophic lake, Esthwaite Water (United Kingdom). It was validated against monitoring data from 2003 and simulated well the seasonal pattern of total chlorophyll, diatom chlorophyll and Cyanobacteria chlorophyll with respective R²‐values calculated between observed and simulated of 0.68, 0.72 and 0.77 (all P<0.01). This simulation was then rerun through various combinations of factorized changes covering a range of half to double the flushing rate and from ?1 to +4 °C changes in water temperature. Their effect on the phytoplankton was measured as annual, spring, summer and autumn means of the total and species chlorophyll concentrations. In addition, Cyanobacteria mean percentage abundance (%Cb) and maximum percentage abundance (Max %Cb) was recorded, as were the number of days that Cyanobacteria chlorophyll concentration exceed two World Health Organization (WHO) derived risk thresholds (10 and 50 mg m^?3). The phytoplankton community was dominated in the year by three of the eight phytoplankton simulated. The vernal bloom of the diatom Asterionella showed little annual or seasonal response to the changing drivers but this was not the case for the two Cyanobacteria that also dominated, Anabaena and Aphanizomenon . These Cyanobacteria showed enhanced abundance, community dominance and increased duration above the highest WHO risk threshold with increasing water temperature and decreasing flushing rate: this effect was greatest in the summer period. However, the response was ultimately controlled by the availability of nutrients, particularly phosphorus and nitrogen, with occasional declines in the latter's concentration helping the dominance of these nitrogen‐fixing phytoplankton.