Low-intensity land use in grassland catchments: Effects on a large, oligotrophic lake

Abstract

In southern New Zealand, many upland streams drain into large oligotrophic lakes surrounded by native grassland, low-intensity farming, and small urban centers. Little work has been undertaken to determine the impact low-intensity development has on nutrient dynamics and microbial activity in these large lake systems. Lake Wanaka, Central Otago, was chosen as a study site since the recent appearance of nuisance organic aggregates and changes in phytoplankton community structure suggest the lake is not in a steady state. Research undertaken for this project included intensive sampling of tributaries to the lake during different seasons and hydrological conditions, following the path of two tributaries out into the lake, and laboratory-based experiments. In the Wanaka catchment, pasture cover correlated positively with stream dissolved organic carbon (DOC), total nitrogen (TN) and nitrate-nitrogen (NO3-N) concentrations. Nitrogen concentrations were not influenced by weather-related variables, but temperature and soil moisture mitigated the influence of pasture cover on surface water DOC concentration under very dry or wet conditions. Neither land use nor weather-related conditions correlated with total phosphorus (TP) or dissolved phosphorus (DRP) concentrations in streams, possibly reflecting good P-binding in soils, low-intensity agriculture in the catchment and/or lack of sampling during high flow events. Amending lake water with stream water in the laboratory did not influence the production of sticky polysaccharides (i.e. transparent exopolymer particles (TEP)), but enriching treatments with high concentrations of N and P increased TEP 1.7 to 9.3 times over unamended treatments. Phytoplankton cell numbers, diatom abundance, and chl a also increased in response to nutrient-enrichment, and organic aggregates were visible in nutrient-enriched treatments within 6 days. In the field, the intermixing depth of a main river inflow varied under stratified and un-stratified conditions, affecting where catchment-derived material was delivered in the Lake. Nutrient and DOC concentrations in the Matukituki River were within range of the Lake, and the river plume was capable of stimulating phytoplankton growth in nearshore waters. Despite similar bulk DOC concentrations, dissolved organic matter (DOM) character and lability differed between the River and the Lake. DOM from deep-sourced lake water contained more aromatic, refractory structures than shallower lake water or river water. The river had almost double the number of organic sulphur compounds than the lake, including potential sulfonates. The source of the S is unknown, but may be geologic in origin or reflect agricultural activity in the River catchment. In the laboratory, riverine bacterial communities could break down a diverse array of organic substances regardless of season, suggesting a consistent labile supply of DOM. In contrast, organic substrate use patterns in the lake were seasonal, and varied by depth. Lake water amended with Matukituki River water stimulated bacterial respiration and uptake of DOC and P, but did not affect bacterial productivity, which may reflect limitations of the experimental design. My results indicate low intensity land use in grassland catchments affects nutrient flux and microbial processes in Lake Wanaka. These data provide a foundation for future research on land development and microbial dynamics in similar large, oligotrophic lake systems.