|dc.description.abstract||Lake Rotorua is a large (80 krn2), polymictic, eutrophic, shalJow (mean depth 10.8 m) lake in central North Island, New Zealand. Blooms of cyanobacteria and occasional anoxia of bottom waters are characteristic of the water quality in the lake during summer months (Dec-Mar.). This study examines the dynamics of internal nutrient recycling processes in Lake Rotorua, including sediment nutrient release, sedimentation and resuspension, phytoplankton nutrient limitation and the relative importance of internal and external nutrient loads on influencing cyanobacteria biomass and summer bloom formation.
Sediment nutrient release rates of phosphorus (P) in Lake Rotorua were estimated from changes in bottom water P concentration during a 19-day summer stratification event in February 2003. Changes in mass of hypolimnion P due to mixing events, together with settling, inflows, diffusion and regeneration, were factored into a simple model which was used to elucidate and quantify the complex nature of phosphorus fluxes in polymictic systems. Sediment soluble reactive phosphorus (SRP) release rates reached values of up to 26.3 mg m-2 d- 1 and increased during stratification, coinciding with a decline in bottom water dissolved oxygen (DO) concentrations.
Phytoplankton nutrient limitation was examined in the lake using three in situ incubation experiments of 4 to 6 day duration in summer 2004. Two of the incubations were conducted during stratification, and one immediately after breakdown of stratification. Samples were enriched with either ammonium ( 1 mg NI-Lt L-1), phosphate (0.1 mg P04 L-1), or with both nutrients. A control with no nutrient addition was used for comparison. Phytoplankton responses to nutrient additions were determined at a species level from cell counts and at a community level from changes in chlorophyll a (chi-a) concentration. A simple phytoplankton growth model was applied to consider the interacting effects of P, nitrogen (N) and light limitation. Phytoplankton biomass generally responded to N plus P additions to a greater extent than with single nutrient additions alone, however, results were often not significant. Increase in community biomass was greater for P than N, and nutrient demand decreased after breakdown of stratification. Individual species responded differently to N and P additions, suggesting co-limitation, and that management of water quality in Lake Rotorua should restrict inputs of both N and P. Model results indicate that light also plays a major role in limiting phytoplankton biomass.
Seasonal variations in sedimentation rates of total particulate matter (TPM), total phosphorus (TP), total nitrogen (TN) and chi-a were measured at three sites in the lake using cylindrical sediment traps. Deployment of traps at different depths at each site showed an increase in sedimentation rates of particulate inorganic material with trap depth, indicating that sediment resuspension is an important process in this lake. Resuspension was estimated to contribute up to 71 % of TPM sedimentation at the shallowest site. Net sedimentation rates across all sites, excluding resuspension fluxes, were 4.5 g m-2 d- 1 for TPM, and 19.8, 103.7 and 42.1 mg m-2 d-1 for TP, TN and chi-a, respectively. Sedimentation rates of all variables were highest in summer and at the deepest sampling site. Mean net sedimentation rates of N and P were between four and nine times greater, respectively, than estimates of net retention based on a nutrient mass balance, demonstrating that internal nutrient recycling is an important process in Lake Rotorua.
Sediment release rates of SRP and Nlli were further determined seasonally at three sites (water depth 7, 14 and 20 m) in Lake Rotorua using in situ benthic chamber incubations. Rates of release of SRP ranged from 2.2 to 85.6 mg P m·2 d"1 and were largely independent of DO concentration. Two phases of Nlli release were observed in the chamber incubations; high initial rates of up to 2200 mg N m-2 d-1 in the first 12 h of deployment followed by lower rates of up to 270 mg N m-2 d-1 in the remaining 36 h of deployment. Releases of SRP and Nai were highest in summer and at the deepest of the three sites. High organic matter supply rates to the sediments may be important for sustaining high rates of sediment nutrient release. A nutrient budget of Lake Rotorua indicates that internal nutrient sources derived from benthic fluxes are more important than external nutrient sources to the lake.
In the final part of this thesis, a vertically resolved water quality model, DYRESMCAEDYM, was used to assist with quantifying the relative contributions of internal and external nutrient inputs to the lake and their relative importance to cyanobacteria bloom formation. External nutrient loads were derived for 26 tributaries as well as for iii groundwater and stormwater flows. The total external load is 534 t yr-1 for TN and 34 t yr-1 for TP. Other forcing inputs to the model included meteorological data collected at a station beside the lake and discharge from the only outflow, Ohau Channel. Measured rates of sediment nutrient release obtained from benthic chamber measurements, profiles of water column nutrient concentrations, surface chl-a concentration and temperature and dissolved oxygen loggers were used to validate output from the DYRESM-CAEDYM model. Simulations of water column temperatures and SRP and NI-Li concentrations in Lake Rotorua showed a close representation of field measurements, and captured the timing and duration of stratification events and subsequent changes in bottom water nutrient concentrations. Model simulations of different nutrient loading scenarios indicate that reductions in sediment nutrient fluxes would be more effective in reducing cyanobacterial biomass than similar reductions in catchment fluxes, due to the incidence of large sediment nutrient release events in association with summer blooms. This finding indicates that only a significant and prolonged reduction in external loads, that in tum would reduce internal loads, will ultimately decrease cyanobacteria biomass in Lake Rotorua.
In this study, field work and modelling were used to demonstrate the importance of internal nutrient loads in sustaining the current trophic status of Lake Rotorua. High external loading rates, coupled with high rates of organic matter sedimentation and sediment nutrient release rates, suggest that high phytoplankton biomass will remain a feature of this lake unless a significant and prolonged reduction in both N and P external loads is undertaken. The results of this study also emphasised the importance of conducting experimental measures of sedimentation and sediment nutrient release rates within eutrophic lakes, particularly where internal recycling processes may represent the dominant sources of nutrients to the lake.||