Novel tests of regular seasonality, types of variability, and modes of succession in lake phytoplankton

Abstract

The changing composition of phytoplankton communities was followed in 2 high latitude temperate lakes, Esthwaite Water and Windermere (North Basin), by a novel presentation method applied to ratings of semiquantitative abundance, distinguishing the most abundant component as “dominant,” with the merits of rapidity and summary potential. Regular or irregular seasonalities were tested by sums of total sampling occurrences in successive weeks-of-year. Results for occurrence ranged from regular seasonality (e.g., the diatom Asterionella formosa) to widely diffused multiseasonal (“opportunistic”; e.g., Cryptomonas spp.) or aseasonal persistence (e.g., Aphanizomenon flos-aquae). Some abundant species occurred in abundance in blocks of adjacent years, between which was apparent discontinuity. Such discontinuity was in part related to known species-specific epidemics of fungal or protozoan parasitism (e.g., on Ceratium spp.) Conversely, dominance in adjacent years was likely to be favoured by over-winter survival in plankton or benthos from a prior year dominant (e.g., Aphanizomenon flos-aquae; Esthwaite Water 1982–1983) and perennating benthic stages such as cysts (Ceratium). Also distinguished were short periods of rapidly changing dominance, often involving cryptomonads, indicative of diverse prior competing subdominants at considerable concentration. One such period was marked by high consumption by Daphnia spp. in an annual clearwater phase.