Environmental and Seasonal Influences on Lake Phytoplankton Community Structure

Habib, Olfat Anwar (1993) Environmental and Seasonal Influences on Lake Phytoplankton Community Structure. MSc(R) thesis, University of Glasgow.

Abstract

This is an investigation into the effects of the environment on seasonal changes of phytoplankton community structure and production, contrasting the north and south basins of Loch Lomond, Scotland. It commenced in mid January 1991 and continued until late May 1993. The north basin of Loch Lomond is a warm monomictic type, showing a single circulation period and is clearly stratified during summer. The south basin is polymictic because it stratifies only for a short period. In the north basin the effect of incident radiation is reduced by the small surface area to volume ratio and the greater amount of cloud cover. The south basin is wide and open with a larger surface area and consequently receives more light energy per unit volume of the water than the north. Dissolved oxygen in the water at both sites never showed significant depletion and is typical of oligotrophic waters. The vertical distribution of oxygen in both basins of Loch Lomond tends to be slightly clinograde during summer and orthograde in winter. Dissolved oxygen concentrations were at nearly saturation and supersaturation levels through the period of isothermal conditions and declined to the minimum in the hypolimnion after several weeks of thermal stratification. The annual range of pH in Loch Lomond is similar in both basins. Alkalinity showed a small annual range generally being high in spring and low in winter. Conductivity shows that there is a much larger dissolved mineral concentration in the south than the north. The pattern of seasonal nutrient variation in Loch Lomond showed a maximum abundance of phosphate, silicate and nitrate occurring in winter and the lowest concentration in the summer. Silicate and nitrate were always detectable. The chemical oxygen demand in the south basin is on average 33% higher than in the north basin. The seasonal succession of phytoplankton in Loch Lomond exhibited a pattern of domination by diatoms in winter and spring. In general, the south basin retained a higher standing crop than the north basin and the growing season usually started earlier and extended over a longer period of time. The chlorophyll a peak always preceded or coincided with the peak of phytoplankter numbers. Although the largest peak for chlorophyll in the south basin was in autumn, the largest peak for numbers was at the end of June. From the TWINSPAN classification and CCA ordination for both basins of Loch Lomond, it was clear that the phytoplankton community in winter and spring was dominated mainly by diatoms, with some blue-greens. At this time nutrients are at their maximum level. Silicate appeared to be more important than nitrate and phosphate as a potential environmental control on phytoplankton at this time of year. The summer and autumn species were probably favoured by increasing temperature and tolerance of lower nutrient availability. Both basins retain 42 species. Diversity is slightly higher in the north than in the south basin except during spring and early summer when values are more or less the same. Melosira italica, considered to be a plankton characteristic of nutrient-rich lakes, was the most dominant species in the south basin during spring and autumn. The oligotrophic Cyclotella kutzingiana , presented the main constituent in the north basin from early spring to early autumn. The decline in phosphate concentration, was correlated with the spring algal increase but it is unlikely that this reached growth limiting levels for diatoms before silicate depletion occurred. Blue-green algae, particularly Merismopedia glauca, an indicator of oligotrophic waters, were more important in the north than the south basin. Dinobryon divergens occurred more in the north basin than in the south basin: this species also prefer nutrient-poor conditions. Both in terms of standing crop and primary production there was a difference in the state of algal population growth during spring between the north and south basins. In the south basin growth started 4-6 weeks earlier than in the north, and reached a peak 3.5 times as high. The seasonal zooplankton pattern displayed marked variations in relation to phytoplankton composition and concentration. The general dominance of rotifers through most of the year was related to their faster rate of reproduction compared with cladocera and copepoda. The seasonal variations of rotifers were also likely to be influenced by temperature and possible supplementary food sources.

• Item Type: Thesis (MSc(R))
• Qualification Level: Masters
• Additional Information: Adviser: Kevin Murphy
• Keywords: Zoology, Limnology, Microbiology
• Date of Award: 1993
• Depositing User: Enlighten Team
• Unique ID: glathesis:1993-74639
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 27 Sep 2019 17:25
• Last Modified: 27 Sep 2019 17:25
• URI: https://theses.gla.ac.uk/id/eprint/74639