Seasonal dynamics of macrophytes and phytoplankton in shallow lakes: a eutrophication‐driven pathway from plants to plankton?

Abstract

Summary 1. Seasonal relationships between macrophyte and phytoplankton populations may alter considerably as lakes undergo eutrophication. Understanding of these changes may be key to the interpretation of ecological processes operating over longer (decadal‐centennial) timescales. 2. We explore the seasonal dynamics of macrophytes (measured twice in June and August) and phytoplankton (measured monthly May–September) populations in 39 shallow lakes (29 in the U.K. and 10 in Denmark) covering broad gradients for nutrients and plant abundance. 3. Three site groups were identified based on macrophyte seasonality; 16 lakes where macrophyte abundance was perennially low and the water generally turbid (‘turbid lakes’); 7 where macrophyte abundance was high in June but low in August (‘crashing’ lakes); and 12 where macrophyte abundance was high in both June and August (‘stable’ lakes). The seasonal behaviour of the crashing and turbid lakes was extremely similar with a consistent increase in nutrient concentrations and chlorophyll‐a over May–September. By contrast in the stable lakes, seasonal changes were dampened with chlorophyll‐a consistently low (<10–15 μg L−1) over the entire summer. The crashing lakes were dominated by one or a combination of Potamogeton pusillus, Potamogeton pectinatus and Zannichellia palustris, whereas Ceratophyllum demersum and Chara spp. were more abundant in the stable lakes. 4. A long‐term loss of macrophyte species diversity has occurred in many shallow lakes affected by eutrophication. One common pathway is from a species‐rich plant community with charophytes to a species‐poor community dominated by P. pusillus, P. pectinatus and Z. palustris. Such compositional changes may often be accompanied by a substantial reduction in the seasonal duration of plant dominance and a greater tendency for incursions by phytoplankton. We hypothesise a slow‐enacting (10–100 s years) feedback loop in nutrient‐enriched shallow lakes whereby increases in algal abundance are associated with losses of macrophyte species and hence different plant seasonal strategies. In turn such changes may favour increased phytoplankton production thus placing further pressure on remaining macrophytes. This study blurs the distinction between so‐called turbid phytoplankton‐dominated and clear plant‐dominated shallow lakes and suggests that plant loss from them may be a gradual process.

Water quality and communities associated with macrophytes in a shallow water-supply reservoir on an aquaculture farm

“…Temperature influences the plankton communities, because phytoplankton and zooplankton may have different temperature optima for growth (Sayder et al, 2010). Mean temperature may indirectly determine the plankton through macrophytes, because the growth status of macrophytes at different temperatures may have different effects on environment conditions (i.e., chemical composition, organic matter) in the water column (Zeng et al, 2012).…”

“…Where the period of macrophyte occupancy is log (such as this study), plant-associated zooplankton and invertebrate grazers may also preclude substancial phytoplankton development (Sayder et al, 2010). Fast ammonia uptake by phytoplankton caused high Zygnematophyceae density.…”

“…The macrophyte S. auriculata in conditions of competition for space, such as high density, tended to reduce its life cycle and produced more sporocarps (Sayder et al, 2010). After the death of plants, decomposition may also release large amounts of nutrients in the environment (Coelho et al, 2005).…”

“…The functional role of macrophytes tends to be greatest when plants fill the entire water column and when the growing season is long. Under eutrophic conditions, however, gaps may occur in plant cover either spatially (macrophytefree areas) or temporally (seasonally or between-years) (Sayder et al, 2010).…”

“…Macrophytes reduce phytoplankton abundance not only through direct competition for nutrients but also through the provision of habitat and invertebrate grazers (Sayder et al, 2010).…”