Alternative equilibria dominated by either submerged plants or phytoplankton have been described for communities in shallow lakes. A nutrient-mediated increase in periphyton (algae attached to plant surfaces) is often described as being responsible for the loss of plants from shallow lakes, yet this violates the stochastic assumptions of alternative equilibria. To determine if periphyton is capable of forcing a switch between these communities, the factors governing the success of aquatic plants were surveyed in 17 plantdominated shallow lakes in Norfolk (United Kingdom) that varied in nutrient concentration and fish biomass. In these lakes, plant biomass was negatively related to the density of periphyton. However, the density of periphyton on the plants was correlated with the density of grazing invertebrates, not nutrient concentration. In turn, the biomass of fish determined the density of invertebrates. This cascade from fish to periphyton via invertebrates appeared to be evident even though plant-dominated lakes are heterogeneous and complex. Under conditions of plant dominance, periphyton appeared to have a stronger influence on plant growth than phytoplankton. Our data support a model where, within the range of nutrients where alternative equilibria are possible, fish are the prime determinants of community structure in shallow lakes, through a cascading effect of predation on grazing invertebrates influencing the biomass of periphyton and hence, plants. We suggest that the stochasticity required for alternative equilibria in shallow lake communities is often derived from the vagaries of fish colonization and reproduction.

With the implementation of the EU Water Framework Directive (WFD), the member states have to classify the ecological status of surface waters following standardised procedures. It was a matter of some surprise to lake ecologists that zooplankton were not included as a biological quality element (BQE) despite their being considered to be an important and integrated component of the pelagic food web. To the best of our knowledge, the decision of omitting zooplankton is not wise, and it has resulted in the withdrawal of zooplankton from many so-far-solid monitoring programmes. Using examples from particularly Danish, Estonian, and the UK lakes, we show that zooplankton (sampled from the water and the sediment) have a strong indicator value, which cannot be covered by sampling fish and phytoplankton without a very comprehensive and costly effort. When selecting the right metrics, zooplankton are cost-efficient indicators of the trophic state and ecological quality of lakes. Moreover, they are important indicators of the success/ failure of measures taken to bring the lakes to at least good ecological status. Therefore, we strongly recommend the EU to include zooplankton as a central BQE in the WFD assessments, and undertake similar regional calibration exercises to obtain relevant and robust metrics also for zooplankton as is being done at present in the cases of fish, phytoplankton, macrophytes and benthic invertebrates.

The conservation of threatened species must be underpinned by phylogeographic knowledge. This need is epitomized by the freshwater fish Carassius carassius, which is in decline across much of its European range. Restriction site-associated DNA sequencing (RADseq) is increasingly used for such applications; however, RADseq is expensive, and limitations on sample number must be weighed against the benefit of large numbers of markers. This trade-off has previously been examined using simulation studies; however, empirical comparisons between these markers, especially in a phylogeographic context, are lacking. Here, we compare the results from microsatellites and RADseq for the phylogeography of C. carassius to test whether it is more advantageous to genotype fewer markers (microsatellites) in many samples, or many markers (SNPs) in fewer samples. These data sets, along with data from the mitochondrial cytochrome b gene, agree on broad phylogeographic patterns, showing the existence of two previously unidentified C. carassius lineages in Europe: one found throughout northern and central-eastern European drainages and a second almost exclusively confined to the Danubian catchment. These lineages have been isolated for approximately 2.15 m years and should be considered separate conservation units. RADseq recovered finer population structure and stronger patterns of IBD than microsatellites, despite including only 17.6% of samples (38% of populations and 52% of samples per population). RADseq was also used along with approximate Bayesian computation to show that the postglacial colonization routes of C. carassius differ from the general patterns of freshwater fish in Europe, likely as a result of their distinctive ecology.

1. Submerged macrophyte and phytoplankton components of eutrophic, shallow lakes have frequently undergone dynamic changes in composition and abundance with important consequences for lake functioning and stability. However, because of a paucity of long-term survey data, we know little regarding the nature, direction and sequencing of such changes over decadal-centennial or longer timescales. 2. To circumvent this problem, we analysed multiple (n = 5) chronologically correlated sediment cores for plant macro-remains and a single core for pollen and diatoms from one small, shallow, English lake (Felbrigg Hall Lake, Norfolk, U.K.), documenting 250 years of change to macrophyte and algal communities. 3. All five cores showed broadly similar stratigraphic changes in macrophyte remains with three distinct phases of macrophyte development: Myriophyllum-Chara-Potamogeton (c. pre-1900), to Ceratophyllum-Chara- Potamogeton (c. 1900Potamogeton (c. -1960 and finally to Zannichellia-Potamogeton (c. post-1960). Macrophyte species richness declined from at least 10 species pre-1900 to just four species at the present day. Additionally, in the final Zannichellia-Potamogeton phase, a directional shift between epi-benthic and phytoplanktonbased primary production was indicated by the diatom data. 4. Based on macrophyte-seasonality relationships established for the region, concomitant with the final shift to Zannichellia-Potamogeton, we infer a reduction in the seasonal duration of plant dominance (plant-covered period). Furthermore, we hypothesise that this change in species composition resulted in a situation whereby macrophyte populations were seasonally 'sandwiched' between two phytoplankton peaks in spring and late summer as observed in the contemporary lake. 5. We suggest that eutrophication-induced reductions in macrophyte species richness, especially if the number of plant-seasonal strategies is reduced, may constrict the plant growing season. In turn, this may render a shallow lake increasingly vulnerable to seasonal invasions of phytoplankton resulting in further species losses in the plant community. Thus, as part of a slow (over perhaps 10-100s of years) and self-perpetuating process, macrophytes may be gradually pushed out by phytoplankton without the need for a perturbation as required in the alternative stable states model of plant loss.