Abstract of PhD Thesis

The relationship between resource elemental deficiencies and zooplankton community structure and dynamics

Valerie McCarthy (2007) - Trinity College Dublin

Ecological stoichiometry is the study of the balance of elements in ecological interactions and processes. Recent theoretical and experimental work suggests that consideration of stoichiometric constraints on zooplankton production may substantially improve our understanding of zooplankton community composition and species interactions. This study aimed to examine the role of stoichiometry in influencing zooplankton community structure, species occurrence and seasonal patterns of succession in six lakes in the west of Ireland. Stoichiometric processes were viewed in the context of the overall biotic and abiotic environment which may also potentially influence zooplankton community structure and dynamics.

This research incorporated two field studies involving six sites located in the west of Ireland. These lakes were separated according to alkalinity type, three high-alkalinity and three low-alkalinity lakes. Daphnia and calanoids dominated zooplankton biomass throughout the year in the high-alkalinity lakes, whereas cladoceran taxa other than Daphnia, such as Holopedium gibberum and Diaphanosoma brachyurum, dominated at certain times of the year in the low-alkalinity lakes. Overall zooplankton community size structure also differed between the two alkalinity groups, with a greater proportion of larger individuals in the high-alkalinity lakes and smaller individuals dominating the size distribution in the low-alkalinity lakes. Lough Feeagh was found to have the most distinctive taxa composition of the low-alkalinity lakes, with relatively high abundances of Daphnia recorded compared with the other low-alkalinity lakes. Lough Talt was also the most distinct high-alkalinity lake, and was quite similar in zooplankton community structure to the low-alkalinity lake, Lough Easky. Both are upland and geographically adjacent lakes. Size distribution analyses and measures of cyclomorphosis in Daphnia were used to assess the role of both vertebrate and invertebrate predation in influencing seasonal patterns of species dominance among the lakes. There was strong evidence of fish predation in all three high-alkalinity lakes and in Lough Feeagh, but was weaker in the other two low-alkalinity lakes. There was evidence of invertebrate predation in most lakes; although such evidence was weaker in Lough Carra and Lough Easky. There is, therefore, evidence which suggests that the physical, chemical and biotic environment was fundamental in structuring the zooplankton assemblages of the six lakes of this study.

An intensive two-year study was carried out in Lough Carra, a shallow marl lake in the west of Ireland. This field study investigated whether population fluctuations of the two dominant taxa, Daphnia spp. and the calanoid Eudiaptomus gracilis, were associated with the availability of phosphorus and nitrogen. According to stoichiometric theory, zooplankters have a species-specific elemental composition. Daphnia have a relatively high phosphorus concentration in their tissues and copepods high nitrogen. Daphnia should, therefore, be more sensitive to phosphorus limitation and copepods more sensitive to nitrogen. In accordance with stoichiometric predictions, there was evidence to suggest that Daphnia and Eudiaptomus reproduction in Lough Carra had contrasting relationships to dietary phosphorus and nitrogen availability. Egg production by Daphnia was associated with measures of phosphorus availability, whereas calanoid egg production was associated with measures of nitrogen availability. Daphnia biomass was not, however, correlated with phosphorus availability, and neither was calanoid biomass correlated with nitrogen. The ratio of DIN:TP was high when Daphnia dominated the zooplankton biomass, and low when calanoids dominated. This pattern is consistent with Daphnia acting as a sink for phosphorus and calanoids as a sink for nitrogen and suggests consumer-driven nutrient recycling.

The role of resource quality in influencing zooplankton community structure across all six lakes of this study was then examined. There was a negative relationship between Daphnia abundance and the seston C:P and a positive correlation was observed between bulk zooplankton C:P content and bulk seston C:P content. This suggests that in accordance with stoichiometric expectations high-P taxa such as Daphnia were replaced by low-P taxa with declining food P content. There were, however, poor associations between seasonal variations in zooplankton dynamics and seasonal fluctuations in nutrient availability in many lakes. This suggests that factors other than resource nutrient limitation, such as fish predation, may have a stronger influence in determining seasonal shifts in taxa dominance. Seston nutrient content, nevertheless, appeared to have some influence in constraining the maximum abundance of high-P taxa such as Daphnia across the six study lakes. It is important to consider the role of stoichiometry as only one of several alternative factors influencing zooplankton community structure and dynamics.