Impacts of climate change and nutrients on phytoplankton, especially on cyanobacteria

Abstract

Eutrophication is a global problem experienced by many water bodies that can lead to excessive growth of phytoplankton, especially cyanobacteria that create numerous water quality issues. Cyanobacterial species are known to produce toxins that pose threats to animal health and can produce large scale surface scums that degrade aesthetics and reduce oxygen in aquatic systems. It is hypothesised that increasing temperatures with climate change would further intensify the effects of eutrophication on phytoplankton dynamics. It is likely that both varying temperatures and nutrients interact to change the growth environment of phytoplankton, although, the relative influence of each factor may vary depending on nutrient availability. Rising temperature and nutrients can affect phytoplankton community directly by changing the metabolic rates and indirectly by altering the thermal structure of lakes. The main aim of this study was to understand the changes in the phytoplankton growth, succession and composition as a result of climate change and nutrients. One main focus was to determine whether the negative impacts of rising temperatures could be alleviated by nutrient reduction. Three primary approaches were taken to address the above aims. First, the response of phytoplankton to varying nutrients under a warming climate was assessed in two lakes with different trophic status. Second, the oligotrophic lake was used as model system to explore the changes experienced by the water column stability as a result of changes in nutrients and temperature. Finally, the effects of water column stability on phytoplankton abundance and functional diversity were assessed by analysing small scale changes in mixing and stratification during the transition period from spring to summer. The importance of trophic status in driving phytoplankton dynamics was determined using two lakes with opposing nutrient status (Chapter 2). The responses and interactions of three main phytoplankton functional groups consisting of cyanobacteria, chlorophytes and diatoms under rising temperatures and modified nutrient availabilities were analysed. A calibrated and validated open-source, 1-dimensional hydrodynamic model, General Lake Model (GLM) coupled with the Framework for Aquatic Biogeochemical Model (FABM) was used to simulate the hydrodynamics, nutrients and phytoplankton in the oligotrophic and eutrophic lake. A combination of 25 scenarios, including changes in nutrient availability and rising temperatures, were used to evaluate and compare the responses of the phytoplankton in the two lakes. Nutrients were decreased and increased by 10-20 folds to cause a shift in the trophic status and temperatures were increased by 1-4 °C based on the future climate projections in the IPCC 4th Assessment Report. Nutrients and temperatures, individually and interactively, were found to cause substantial changes in the phytoplankton growth and composition. A 3-fold increase in the phytoplankton biomass relative to the base conditions was observed in the oligotrophic lake as the nutrient status changed from meso-oligotrophic to hyper-eutrophic. The effects of rising temperatures were disguised by the profound impacts of nutrient enrichment in the oligotrophic lake. The eutrophic lake also experienced a 2-fold increase in phytoplankton growth under high nutrients, but rising temperature had a higher influence on phytoplankton abundance. Reduced nutrients had a minor influence on the total phytoplankton biomass in the oligotrophic lakes but, in the eutrophic lake, it prevented the boosted phytoplankton growth which would have otherwise occurred under high temperatures. Nuisance phytoplankton growth expected under high temperatures can be alleviated by nutrient reduction. Phytoplankton composition also changed as the nutrient status changed under a warming climate. Chlorophytes were observed to dominate under high nutrients and cyanobacteria thrived under low nutrient and high temperatures in both lakes. Although cyanobacteria species are normally observed under eutrophic, warm conditions, the physiological adaptations of chlorophytes and cyanobacteria that promote their functionality, such as their high surface area to volume ratios, presence of mucilaginous sheath and resistance to predation, could explain the above observations. A simple sensitivity analysis indicated that trait specific parameters related to growth and nutrient uptake rates have a considerable influence on the interactions simulated between chlorophytes and cyanobacteria. The thermal structure of lakes is an important physical feature that exerts control over the ecosystem structure and function. The changes in the mixing and stability of the water column in the oligotrophic lake as a consequence of varying nutrients and rising temperatures were then assessed in detail in Chapter 3. Similar scenarios were adopted and the resulting thermal profiles were used to calculate the thermal stability and thermocline depth for each scenario. Lake Tarawera experienced atypical variations in the seasonal heat fluxes under nutrient rich, warm scenarios. The highest evaporative losses occurred in spring-summer period and an increase in net heat occurred during winter-autumn period. The latter caused an advancement in the onset of stratification as the trophic status changed from oligotrophic to hyper-eutrophic under rising temperatures. The onset and duration of stratification were assessed and the relative contribution of surface temperature, nutrients and mixing in determining thermal stability under each scenario was evaluated using linear regression models. A rise in temperature by 1-4 °C under eutrophic conditions led to an increase in average thermal stability of the water column with the onset of stratification events advancing and persisting for a longer period of time relative to base conditions. Under similar circumstances, the average thermocline depth was also reduced, and persistent, incomplete mixing was observed, even during winter. Temperature, thermocline depth and total chlorophyll were the main factors that influenced thermal stability but the influence of each factor varied depending on the nutrient status of the lake. Nutrient driven total phytoplankton concentrations had the highest influence on stability under intermediate temperature rises of 1-2 °C. Beyond a temperature rise of 2 °C, temperature had the greatest impact on stability of the water column. Nutrient reduction was found to improve winter mixing by increasing thermocline depth under a temperature increase of 2 °C, which was absent during nutrient enriched conditions under the same temperature rise. Hence, nutrient reduction can increase mixing and improve water quality in deep lakes under temperature increases between 1 and 2 °C, but may not be effective if the temperatures are to rise beyond 2 °C. Changes in mixing, stratification and subsequent modifications in light and nutrient distribution can be considered as disturbances in the ecosystem that have an impact on the phytoplankton abundance and composition. Intensive, twice a week sampling was carried out in Mt Bold Reservoir (largest storage reservoir in South Australia) to determine the changes in the phytoplankton community and the functional diversity as environmental factors vary during the seasonal shift from spring to summer (Chapter 4). Functional diversity of the phytoplankton community was assessed using characteristic morphological adaptations that determine their competitive abilities and survival opportunities. Long term seasonal trends in increasing surface water temperatures and thermal stability emerged as the most significant factors that affect phytoplankton taxonomic and functional diversity. While the taxonomic diversity indicated a progressive declination as spring extends, the functional diversity showed the highest diversity during early spring and then declined steadily. Relating the functional diversity trend to the intensity and the frequency at which the disturbances occurred in the system suggested that the functional diversity fitted the Intermediate Disturbance Hypothesis (IDH) (Grime 1973, Cornell 1978). The highest functional diversity was observed when the magnitude of the forcing event (change in mixing) was of medium intensity when comparing to the minimum and maximum intensities observed in the reservoir, during the period of study. Low intensity fluctuations and gradual increase in stability resulted in the selection of a few genera consisting of Microcystis (cyanobacteria) and Sphaerocystis (chlorophyte) that possessed physiological adaptations to dominate under such conditions. High intensity oscillations in other physical attributes, such as in Lake Number and surface mixed layer created an unstable environment with non-equilibrium conditions that prevented competition exclusion of species and facilitated co-existence amongst species. Overall, this study assessed the direct and indirect impacts of long term changes in the environment in relation to varying nutrient dynamics and climate change and evaluated in detail the effects of short term changes in the abiotic and biotic factors on phytoplankton biomass and diversity. The results from this study highlighted that improved water quality (e.g., reduced nuisance algal growth and cyanobacterial dominance, increased water clarity) in a future climate with increased air temperatures can only be achieved if external nutrient loadings to lakes are reduced considerably. While it is true that climate change demands a global response, local managers have the opportunity to offset some of the impacts through catchment management and reducing nutrients at the local and catchment scale. Nutrient concentrations may have to be reduced substantially from present day values in many lakes if lake resilience to the detrimental impacts of climate change is to be promoted.