Community respiration in lakes: understanding habitat contributions along a trophic gradient and nutrient limitations in a tropical lake

Limnological studies of carbon flux in lakes have recently increased, due to increasing interest in the role of lakes in the global carbon budget. Carbon fluxes, including the metabolic processes of primary production and respiration, occur in all lake habitats. However, whole-lake metabolism studies are disproportionately representative of the pelagic habitat, and often ignore the littoral benthic habitat. The littoral benthic habitat is a highly productive, yet patchy system, which contributes to the metabolic balance of an entire lake. This study focuses on community respiration in pelagic and littoral benthic habitats: 1) through a comparison of their relative contributions in lakes along a trophic gradient, and 2) through an investigation of stimulatory effects of inorganic nutrients and organic substrates in a deep, tropical lake which exhibits signs of recent cultural eutrophication.Chapter one includes an introduction to the project. The objectives of Chapter two were to compare the magnitude, variability, and relative contributions of dark community respiration: 1) between habitats (littoral vs. pelagic), and 2) in lakes along a trophic gradient (ultra-oligotrophic to hyper-eutrophic). Community respiration rates were measured using biochemical oxygen demand techniques during a dark incubation period in a temperature controlled environment. Water from the pelagic habitat was sampled for concentrations of total phosphorus, dissolved organic carbon, and chlorophyll-a. The results show that community respiration was higher and more variable in the littoral habitat than in the pelagic habitat, in all lakes along the trophic gradient. Additionally, the relative contribution of pelagic community respiration to whole-lake community respiration increased with an increase in trophic state (defined by total phosphorus and dissolved organic carbon levels). The objectives of Chapter three were to: 1) examine differences in magnitude and variability of community respiration and bacterial cell abundance between pelagic and littoral habitats; 2) determine the influence of wet, dry and transitional seasons on community respiration in pelagic and littoral habitats; 3) determine whether community respiration, in pelagic and littoral habitats, is limited by one or more of the following treatment additions: nitrogen, phosphorus, nitrogen and phosphorus, carbon as glucose; and 4) quantify the influence of watershed inputs (sewage, streambed soil, onion field soil, fallow field soil, forest soil, cornfield soil) on pelagic community respiration rates. Dark bioassays were conducted during three different seasons to compare the response of community respiration to inorganic nutrient treatments, and organic substrate treatments that mimic watershed inputs. Community respiration rates were measured using biochemical oxygen demand techniques over a dark incubation period in a temperature controlled environment. The results show that community respiration was higher and more variable in the littoral habitat than in the pelagic habitat of Lake Atitlán. In the pelagic habitat, community respiration was stimulated by glucose, four of the five soil treatments, and sewage. In the littoral habitat, community respiration was stimulated by sewage. We did not find inorganic nitrogen or phosphorus limitation, or co-limitation of inorganic nitrogen and phosphorus, in either habitat. Chapter four includes a conclusion to this thesis and a general summary