The ecology of the freshwater phytoplankton — communities of freely-dispersed, plant-functioning microbes — has attracted attention in Britain for over 100 years. Here an outline history is given, with reference to persons and places, types of water-body involved, and the approaches adopted. The last include taxonomic resolution and composition with range of morphotypes; physiology of growth, photosynthesis and nutrition in relation to environmental variables; population sampling, dynamics and controlling factors; experimental mesocosms; and synecological aspects of communities. Reductionist and autecological approaches have been accompanied by more abstract generalisation, including phytoplankton viewed as a model community.

In 2010, the ‘killer shrimp’ Dikerogammarus villosus (Crustacea: Amphipoda) invaded the British Isles. Past research from central Europe has shown this eastern European shrimp invader to be a ‘voracious omnivore’, highly predatory of a wide range of freshwater macroinvertebrate taxa and also fish fry. It can become ‘super-abundant’ within invaded sites, greatly dominating native assemblages in terms of numbers and biomass. Although the vast majority of past research has focused on the negative impacts of D. villosus invasion on native biodiversity, we consider the usually overlooked implications for biological water quality monitoring and ecological assessment. We show how past invasions of other freshwater shrimp in the British Isles, such as Gammarus pulex and Crangonyx pseudogracilis, have undermined the ability of biotic indices to reliably reflect changes in water quality. Within such invasions, more pollution tolerant invaders can replace more sensitive natives and invaders can be highly predatory of other macroinvertebrate taxa which contribute to biotic indices. We predict the impacts of the D. villosus invasion will be greater than any previous shrimp invasion of the British Isles and indeed potentially of any other freshwater macroinvertebrate invasion thus far. As it spreads throughout the British Isles, we predict this species will have drastic deleterious impacts on native macroinvertebrate assemblages, especially in its preferred rocky/stony habitats. We consider ways forward for future biological water quality monitoring and ecological assessment within D. villosus invaded watercourses.

Cross-ecosystem fluxes are ubiquitous in food webs and are generally thought of as subsidies to consumer populations. Yet external or allochthonous inputs may in fact have complex and habitat-specific effects on recipient ecosystems. In lakes, terrestrial inputs of organic carbon contribute to basal resource availability, but can also reduce resource availability via shading effects on phytoplankton and periphyton. Terrestrial inputs might therefore either subsidise or subtract from consumer production. We developed and parameterised a simple model to explore this idea. The model estimates basal resource supply and consumer production given lake-level characteristics including total phosphorus (TP) and dissolved organic carbon (DOC) concentration, and consumer-level characteristics including resource preferences and growth efficiencies. Terrestrial inputs diminished primary production and total basal resource supply at the whole-lake level, except in ultra-oligotrophic systems. However, this system-level generalisation masked complex habitat-specific effects. In the pelagic zone, dissolved and particulate terrestrial carbon inputs were available to zooplankton via several food web pathways. Consequently, zooplankton production usually increased with terrestrial inputs, even as total whole-lake resource availability decreased. In contrast, in the benthic zone the dominant, dissolved portion of the terrestrial carbon load had predominantly negative effects on resource availability via shading of periphyton. Consequently, terrestrial inputs always decreased zoobenthic production except under extreme and unrealistic parameterisations of the model. Appreciating the complex and habitat-specific effects of allochthonous inputs may be essential for resolving the effects of cross-habitat fluxes on consumers in lakes and other food webs.

Groundwater-adapted species (known as stygobites) provide animportant contribution to biodiversity. Groundwater ecosystems are some of the oldest on earth, and contain many endemic species adapted to live in an environment with no light and limited resources. The controls on stygobite distributions are not yet fully resolved because of the complex interaction between many processes operating at different scales. Many of these processes are geological or hydrogeological in nature and therefore more detailed geological and hydrogeological studies could provide improved understanding of stygobite distributions. Hydrogeologists can assist ecologists by providing expertise on both general geological characteristics of sampling sites, and how groundwater at sampling sites relates to the wider aquifer setting. Geological input would be especially useful in stygobite dispersal studies because dispersal depends upon habitat continuity associated with geological dispersal corridors, and is limited where rocks that do not provide a suitable habitat form geological barriers. Stygobite studies are of benefit to hydrogeology because stygobite distributions can provide information on ground-water-surface water interaction and aquifer connectivity over a range of spatio-temporal scales. Future studies using DNA analysis of stygobites may provide much more detailed information on hydraulic connectivity within and between aquifers. There is also potential for the development of stygobites as indicators of groundwater quality. The biogeochemical function of stygobites is of interest to both hydrogeologists and ecologists. Studies have demonstrated that stygobites graze biofilms and bacteria but their role in biogeochemical cycles is still not fully understood. Ecosystem services provided by groundwater fauna depend upon their abundance and biomass. Future studies using hydrogeological data (e.g. borehole packer techniques) may provide an improved understanding of where in aquifers stygobites live and how many there are, which would be an important step towards assessing the significance of their role in biogeochemical cycling of nutrients and carbon.