Long‐term dynamics of submerged macrophytes and algae in a small and shallow,eutrophic lake: implications for the stability of macrophyte‐dominance

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. 1900–1960) 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 phytoplankton‐based 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.