Nutrient and phytoplankton dynamics of the Hunter River estuary

Observational studies and nutrient amendment experiments were conducted to better understand the nutrient and phytoplankton dynamics of the Hunter River estuary. Eutrophic conditions above ANZECC guidelines for estuaries dominate the Hunter River estuary. The upper Hunter estuary, upstream of its confluence with the Williams River, had the highest concentrations of nutrients and chlorophyll a. The major source of nutrients appears to be riverine discharge. Discharge from WWTP in the upper Hunter potentially contributes an important secondary source of phosphorus. Processes such as bank erosion and resuspension may also be important in explaining variation in nutrient concentrations. Light and turbidity were the main factors limiting phytoplankton growth in the upper estuary. The nutrient amendment experiments showed that when light limitation was alleviated, phytoplankton were either nitrogen limited or remained unlimited by nutrients (suggesting nutrients were in surplus for growth). The expression of nitrogen limitation is likely due to low N:P in the estuary. Organic nitrogen dominates the nitrogen pool within the Hunter estuary. The bioavailability of organic nitrogen in the estuary is unknown which may explain the lack of relationship between phytoplankton and nitrogen concentrations within the estuary. Diatoms and green algae dominated phytoplankton. There were occasions when toxic cyanobacteria was in high abundance in the upper estuary, however a longer data set of phytoplankton assemblage is needed to more adequately assess the risk of toxic cyanobacteria. Comparison of data from the monthly, twice-weekly, and hourly sampling intervals demonstrated the five-year monthly sampling data appeared to mostly capture the variability of nutrient and chlorophyll a concentrations in relation to their main explanatory factors (discharge and light). There were some examples of chlorophyll a and nitrogen concentrations that fell outside of predicted ranges. Overall the results suggest any increase in nitrogen loads to the estuary may lead to increased phytoplankton growth. Improved light climate may also lead to increased phytoplankton growth. Reducing inputs of both nitrogen and phosphorus to the upper Hunter estuary should be a priority action to increase ecosystem health.

Regional-scale phytoplankton dynamics and their association with glacier meltwater runoff in Svalbard

Arctic amplification of global warming has accelerated mass loss of Arctic land ice over the past decades and lead to increased freshwater discharge into glacier fjords and adjacent seas. Glacier freshwater discharge is typically associated with high sediment loads which limits the euphotic depth, but may also provide surface waters with essential nutrients, thus having counter-acting effects on marine productivity. In-situ observations from a few measured fjords across the Arctic indicate that glacier fjords dominated by marine-terminating glaciers are typically more productive than those with only land-terminating glaciers. Here we combine chlorophyll a from satellite ocean colour, an indicator of phytoplankton biomass, with glacier meltwater runoff from climatic mass-balance modelling to establish a statistical model of summertime-phytoplankton dynamics in Svalbard (mid-June to September). Statistical analysis reveals positive spatiotemporal association of chlorophyll a with glacier runoff for 7 out of 14 primary hydrological regions. These regions consist predominantly of the major fjord systems of Svalbard. The adjacent land areas are characterized by a wide range of total glacier coverage (35.5 % to 81.2 %) and fraction of marine-terminating glacier area (40.2 % to 87.4 %). We find that an increase in specific glacier-runoff rate of 10 mm water equivalent per 8-day timeperiod raises summertime chlorophyll a concentrations by 5.2 % to 20.0 %, depending on region. During the annual peak discharge we estimate that glacier runoff contributes to 13.1 % to 50.2 % increase in chlorophyll a compared to situations with no runoff. This suggest that glacier runoff is an important factor sustaining summertime phytoplankton production in Svalbard fjords, in line with findings from several fjords in Greenland. In contrast, for regions bordering open coasts, and beyond 10 km distance from the shore, we do not find significant association of chlorophyll a with runoff. In these regions, physical ocean and sea ice variables control chlorophyll a, pointing at the importance of a late sea ice breakup in northern Svalbard, as well as the advection of Atlantic water masses along the West Spitsbergen Current for summertime phytoplankton dynamics. Our method allows for investigation and monitoring of glacier-runoff effects on primary production throughout the summer season and is applicable on a Pan-Arctic scale, thus complementing valuable but scarce in-situ measurements in both space and time.

An assessment of the light and nutrient regimes constraining phytoplankton dynamics in Missouri reservoirs

Worldwide, nutrient pollution, or eutrophication, is one of the most pervasive environmental issues threatening water quality. Anthropogenic influences, primarily urbanization and agriculture, have drastically increased inputs of bioavailable nutrients to surface water resources, such as lakes and reservoirs. Excessive phosphorus (P) and nitrogen (N) in these waters amplify the growth of suspended algae, or phytoplankton, resulting in unsightly, sometimes odorous, cyanobacterial harmful algal blooms (CyanoHABs) that further degrade water quality and potentially endanger human and animal health. As global climate change increases surface water temperatures, promotes water column stability, and drives down bottom-water oxygen concentrations, environmental conditions are also becoming more favorable for CyanoHABs. With eutrophication prevailing and CyanoHABs increasing in frequency, intensity, and distribution around the globe, I seek to further understand the role of light and nutrients as limiting agents for phytoplankton biomass and primary productivity across Midwestern reservoirs. I utilize numerous lines of evidence to create a robust assessment exploring the influences of climate, eutrophication, and land-use on the proximate light and nutrient status of phytoplankton in 32 Missouri reservoirs. Through observation and experimentation, deficiencies of light, P, and N are evaluated using general indicators of water quality and physiological stress, including mixed layer irradiance, nutrient stoichiometry/debts, photosynthetic efficiency, and the photosynthetic-irradiance (P-E) parameters. Ultimately, I determine if phytoplankton biomass and productivity are constrained by light, P, N, or a combination thereof, across gradients of trophic status and land-use within the context of two contrasting wet and dry summers. As expected, higher proportions of agricultural land-use correspond with higher total in-reservoir nutrient concentrations. Despite agricultural prevalence, however, bioavailable N concentrations in the mixed layer are, overall, relatively low. Yet, P-deficiency is more prominent than either N- or light-deficiency. For the 2018 season, I estimate nearly half of all samples to be P-deficient, with fewer than 20 percent suggesting alternative deficiency or co-deficiency combinations, and approximately one-third indicating sufficiency in both light and nutrients. Primary productivity demonstrates negative relationships with nutrients, biomass, and turbidity, and positive relationships with light. Thus, productivity is highest in clear, low-nutrient reservoirs where light utilization efficiency is also highest. Overall, in Missouri reservoirs, phytoplankton biomass and primary productivity are constrained by P and light, respectively. If current conditions in Missouri reservoirs are at all indicative of those to come as surface waters are further affected by climate change and eutrophication, both P and light will be important regulators of phytoplankton dynamics and subsequent water quality. Contributing to the ongoing P vs NP nutrient management debate, these results both support and challenge aspects of the traditional P-paradigm of limitation on phytoplankton dynamics. It underscores the importance of P control in reservoirs, while offering support for additional consideration of light and N. Having critical implications for watershed management throughout the region, these results are particularly useful within watersheds experiencing high agricultural nutrient loading. Results inform resource managers seeking to employ more effective strategies to control phytoplankton biomass, avoiding harmful regime shifts and CyanoHAB development. Additionally, results may inform lawmakers and regulators developing policies and standards to mitigate nutrient pollution and its effects on water quality at the local, regional, and, potentially, global scales.