Microalgal plankton composition in shallow coastal inlets in contrasting trophic and alternative community states

As the trophic state of the environment changes, communities develop into divergent states. These community states are conventionally reflected through primary producers, because they are directly affected by nutrient availability. Studies of submerged macrophytes often focus on community composition to decipher the vegetative (community) state of the environment, while planktic microalgae are usually viewed more cursorily. Although microalgal plankton composition has been related to the trophic state of shallow temperate lakes, corresponding qualitative knowledge is lacking for shallow inlets in the sea. We assessed the composition of microalgal plankton in relation to that of submerged macrophytes in shallow inlets in the northern Baltic Sea during one ice-free season. Microalgal plankton composition varied distinctively among inlets in different trophic and vegetative states especially during early and mid-season, before becoming comparably uniform. These patterns were consisted both inside and outside of macrophyte beds and during day and night. Local and diurnal variation was comparably high in eutrophic and charophyte-dominated inlets, but only during early season. Microalgal plankton composition not only reflects the state of littoral communities in varying trophic conditions, but it may also be important for the whole trophic structure of those communities.