Factors influencing the spatial distribution of phytoplankton in a small productive lake

(1) A study of the horizontal and vertical distribution of the phytoplankton in a small productive lake, Esthwaite Water in the Lake District, was made during thermal stratification. Distribution patterns of the phytoplankton were related to horizontal variations in specific conductance and pH and to the vertical distribution of temperature and oxygen. (2) A diel study of the changes in the vertical distribution of the dominant algal species, chlorophyll a, temperature and oxygen was also made at the time of maximum phytoplankton density. (3) Appreciable non-uniform horizontal distribution of the phytoplankton was observed only when buoyant blue-green algae or the dinoflagellate Ceratium hirundinella were present. (4) On most occasions horizontal variations of phytoplankton density showed clear systematic patterns related to wind-induced water movements. These patterns were particularly pronounced when individual species formed well-defined vertical aggregations in regions of upwelling or downwelling water. (5) Observations on the vertical distribution of Ceratium showed that this alga underwent diel vertical migrations, with a proportion of the population aggregating at the metalimnion at night. During calm weather large daytime sub-surface accumulations of the alga were found. Vertical perturbations of such small-scale, vertical aggregations may have produced the extreme small-scale horizontal variations of algal concentration noted on two occasions. (6) Daily mean wind speeds above 100 km day-1were normally sufficient to break down patches of phytoplankton, whereas values below 50 km day-1allowed the formation of dense, small-scale, near-surface aggregations of Ceratium. (7) Lateral transport of algal cells at the metalimnion by water currents could assist in maintaining high nutrient gradients at the cell surface, and contribute to the sustained growth of the phytoplankton.