An analysis of the interactions between pelagic food web processes and element cycling in lakes. While some findings are examined in terms of classical concepts from the ecological theory of predator-prey systems, special emphasis is placed on exploring how stoichiometric relationships between primary producers and herbivores influence the stability and persistence of planktonic food webs. The author develops simple dynamic models of the cycling of mineral nutrients through plankton algae and grazers, and then goes on to explore them both analytically and numerically. The results thus obtained are of great interest to both theoretical and experimental ecologists. Moreover, the models themselves are of immense practical use in the area of lake management.

Two of the most powerful ways in which humans have altered ecosystems are by increasing productivity and changing the densities of important consumers. The bottom-up effects of productivity and the top-down effects of consumers have been identified as primary determinants of biological diversity, though the links between them remain unclear. Understanding how consumers and productivity act and interact to yield differences in diversity is of both conceptual and pragmatic importance. Here, I describe three experiments designed to examine the links between grazing, productivity, and diversity in rocky intertidal systems in Oregon and New Zealand. In two experiments I used fully-factorial designs in which I manipulated both grazing and nutrients. Both experiments revealed the primary importance of grazing as a structuring force of algal assemblages. In the Oregon experiment, I also document an interaction between grazing and productivity, with nutrient enrichment decreasing algal diversity at low grazer densities and increasing algal diversity at high grazer densities. This interaction was not apparent in the New Zealand experiment. In the absence of grazers, nutrient addition led to increased abundance of foliose algae at this site but had no net effect on algal diversity. In the third experiment, also conducted in New Zealand, I used natural variation in nearshore productivity as a backdrop against which I manipulated the access of grazers. In this experiment, I found that grazers had negative impacts on benthic algal diversity and abundance at sites with lower productivity and negligible impacts on benthic algal assemblages at sites with higher productivity. Overall these three studies suggest that in these intertidal grazer-dominated communities, the strong top-down effects often documented in such systems can be modified by more subtle bottom-up effects. Together, results from these experiments elucidate factors that determine algal diversity in these systems and underscore the importance of the evolutionary context in which experiments are conducted. Finally, I conclude with a synthesis of the literature in which I put these and other findings to work by exploring the ways in which basic marine ecological research can inform the management of human activities that affect the marine environment.

Nutrient enrichment is common in Irish streams yet despite extensive reports of algal nutrient requirements, generally, this study found no differences in algal biomass across a wide nutrient gradient. In addition, increases in light levels resulted in no net change in algal biomass. Nevertheless, greater densities of grazing benthic macroinvertebrates (grazers), principally Baetis rhodani, were found in high nutrient, high light streams. The ability of grazers to effectively prevent the accrual of highly productive algae was demonstrated through a series of invertebrate grazer exclusion experiments. These experiments also revealed greater grazing pressures at more nutrient rich sites, presumably a function of greater productivity of the grazers{u2019} algal food source. Top-down grazer control of algae was also shown to be highly variable in time and space. Vigorous algal growth during periods of reduced grazing pressure resulted in rapid accrual of the nuisance filamentous alga Cladophora glomerata. Established filaments developed a degree of grazer resistance and there was evidence that these filaments benefitted from the presence of microphyte grazers, suggesting that grazing pressure can shift between a negative and a positive force depending on algal morphology. High-flow events overrode all other interactions and effectively reduced previously accrued algal biomass to levels near zero yet did not negatively effect grazers. Post-spate algal-invertebrate interactions were subsequently reverted to grazer top-down control. During a prolonged low-flow period, Cladophora in shaded habitats accrued to similar levels as in unshaded habitats. These results indicate that despite light-limited growth, algal biomass can attain similar levels over time in shaded habitats, the threshold of which is governed by flow conditions. This study has demonstrated the dominant roles that small bodied grazers and hydraulic disturbance play in regulating algal standing stock in streams in south-west Ireland. These aspects of stream ecosystem function deserve greater attention from a stream management perspective.

This book presents an in-depth discussion of the biological and ecological geography of the oceans. It synthesizes locally restricted studies of the ocean to generate a global geography of the vast marine world.Based on patterns of algal ecology, the book divides the ocean into four primary compartments, which are then subdivided into secondary compartments. *Includes color insert of the latest in satellite imagery showing the world's oceans, their similarities and differences*Revised and updated to reflect the latest in oceanographic research*Ideal for anyone interested in understanding ocean ecology -- accessible and informative

Addresses reservoirs as unique ecological systems and presents research indicating that reservoirs fall into two or three highly concatenated, interactive ecological systems ranging from riverine to lacustrine or hybrid systems. Includes some controversial concepts about the limnology of reservoirs, which make for interesting reading.