Seasonal Water Column NH4 + Cycling Along a Semi-arid Sub-tropical River–Estuary Continuum: Responses to Episodic Events and Drought Conditions

Microbial nitrogen nutrition links to dissolved organic matter properties in East African lakes

East African lakes, especially soda lakes, are home habitats for massive numbers of wildlife such as flamingos, mammals, and fishes. These lakes are known for their high primary production due to local high temperatures, light intensities, and alkalinity (inorganic carbon). However, these lakes, normally within remote areas, receive low nutrient inputs. Ammonium (NH4+) recycling and/or nitrogen fixation can become the major N supply mechanisms for phytoplankton. However, the driving forces on microbial N nutrition in lakes with minimal anthropogenic disturbance remain poorly understood. Using stable isotope tracer techniques, NH4+ recycling rates were measured in 18 lakes and reservoirs in East Africa (Tanzania and Kenya) during the dry season in early 2020. Three functional genes (nifH, gdh, and ureC) relating to microbial N nutrition were also measured. The regeneration of NH4+ supported up to 71 % of the NH4+ uptake. Positive community biological NH4+ demands (CBAD) for all lakes and reservoirs indicate an obvious N demand from microbial community. Our study provides clear evidence that microbial NH4+ uptake rates linked closely to the dissolved organic matter (DOM) properties (e.g., the absorption coefficient at 254 nm, percents of total fluorescence intensity contributed by microbial humic-like and protein-like components) and that water residence time drives microbial NH4+ recycling by regulating the duration of in-lake DOM processing and influencing algal growth. Phytoplankton, especially those of Cyanophyceae, showed maximum biomass and higher NH4+ recycling rates at a certain range of water residence time (e.g., 5-8 years). However, CBAD showed a decreasing trend with longer water residence time, which may be influenced by changes in the algal community composition (e.g., % Cyanophyceae vs. % Bacillariophyceae). These results indicate that DOM dynamics and the water residence time have the potential to facilitate the understanding of microbial nitrogen supply status in East African lakes.

Uncovering nutrient regeneration, transformation pattern, and its contribution to harmful algal blooms in mariculture waters

The prevalence of harmful algal blooms (HABs), especially in mariculture waters, has become a concern for environmental and human health worldwide. Notably, the frequent occurrence of HABs relies upon a substantial supply of available nutrients, which are influenced by nutrient recycling. However, nutrient regeneration, transformation pattern, and their contribution to HABs in mariculture waters remain largely unknown. In this study, by combining field investigation and incubation experiments from June to September 2020, the temporal variations in nutrients and algal composition were revealed. In addition, the nutrient regeneration and assimilation rates in the water column during two continuous algal blooms were measured. The results indicated that organic nutrients, which were the dominant components, strongly stimulated nutrient regeneration. High regeneration rates were observed, with dissolved inorganic nitrogen (DIN) and phosphorous (DIP) regeneration rates ranging from 0.25 to 2.64 μmol/L·h and 0.01 to 0.09 μmol/L·h, respectively. Compared to the direct uptake of organic nutrients, the rapid regeneration of inorganic nutrients played a vital role in sustaining continuous algal blooms, as regenerated DIN contributed 100 % while regenerated DIP contributed 72-100 % of the algal assimilation demand. Furthermore, the redundancy analysis and inverse solution equations indicated that different N transformation patterns and utilization strategies occurred during Heterosigma and Nannochloris blooms. The shorter N recycling pathway and faster NH4+ supply rates provided favorable conditions for the dominance of Nannochloris over Heterosigma, which had a preference for the uptake of NO3-. In conclusion, we propose that nutrient regeneration is a key maintenance mechanism underlying the maintenance of continuous algal blooms, and different N transformation patterns and utilization strategies regulate algal communities in mariculture waters.

Stable hydrogen and oxygen isotopes reveal aperiodic non-river evaporative solute enrichment in the solute cycling of rivers in arid watersheds

We investigated the spatial and temporal variations of the stable isotope composition of hydrogen (δD) and oxygen (δ¹⁸O) and the total dissolved ions (TDI) concentrations in the Okavango River in the middle Kalahari Desert. We aimed to elucidate the role of evaporation in controlling river solute enrichment from samples collected at a one- to two-month frequency from nine stations along a ∼460 km river transect for one year. We found that the δD and δ¹⁸O composition and the TDI concentrations increased downriver. Seasonal increases in the δD and δ¹⁸O composition and TDI concentrations during the hot, rainy season were subdued or decreased during the cool, dry season from pulse flooding. The δD and δ¹⁸O values of the samples plot along the Okavango Delta Evaporation Line consistent with evaporation. The effect of evaporation during river transit was related to the mean δD (δD = 0.07*River distance (km) - 37.9; R² = 0.98) and mean d-excess (d-excess = -0.04*River distance (km) + 9.9; R² = 0.94). The effect of evaporation on the river solute behavior is characterized by the mean d-excess and TDI concentrations (d-excess = -0.29*TDI (mg/L) + 15.0; R² = 0.97). Some samples from this study and those compiled from published studies plot at greater than one sigma standard deviation above and below the mean TDI concentration vs. mean d-excess regression model line. We use these marked deviations from the mean TDI concentration vs. the mean d-excess regression model to suggest that additional solutes from river-floodplain-wetland-island interaction driven by pulse flooding are delivered into the river. While our findings support an evaporation-dominated solute enrichment during river transit at the seasonal scale, we conclude that intermittent hydrology (pulse flooding) plays an important role in the sub-seasonal spatiotemporal behavior of solutes in rivers in arid watersheds, which must be considered in solute cycling models.

Multivariate assessment of outflow water quality in highly urbanized creek system: implication of natural and anthropogenic processes

Nutrient source and transport study in tropical creeks adjacent to megacities are sparse on a regional and global scale. High-frequency chemical data collected during 2017-2018 in the Thane creek system, the largest micro-tidal urbanized creek in Asia, were analysed to assess the chemical water quality, with its linkage to different hydrological stages (southwest monsoon, post- and pre-monsoon) and ongoing anthropogenic activities. Cluster analysis indicates typical zonation between creek outflow and nearshore waters with distinctive physicochemical properties. The creek outflow upholds substantial amounts of nutrient and suspended sediment due to turbid water movement from the ephemeral mudflats and anthropogenic dredging. The year-round hyper-turbid condition in the bottom water triggers the addition of disproportionate nutrients (9% dissolved inorganic nitrogen (DIN) and 14% reactive phosphorous (PO₄^3-) in the outflow region. The DIN and PO₄^3- removal up to 10 and 35%, respectively, occurs in the nearshore region; sedimentation, which acts as a sinking interface for nitrogen and phosphorous, also causes shifting in their limiting conditions. The hyper-turbid condition causes removal of dissolved silicate (DSi) by 5% in the entire region. Ammonium (NH₄⁺) is mainly sourced from the sewage in outflow waters and efficiently mineralized. Chemical indexing of water highlights that the bottom water column remained un-supportive to the balanced ecology. The findings of this study have implications for other tropical creek-estuary concerning management strategies against inadequate flushing. The stalled nutrient export affected balance ecology in tropical Thane creek, which need attention in order to accurately understand the impact on adjacent marine environment and to form effective mitigation policies.

Hydroclimatic variability drives submarine groundwater discharge and nutrient fluxes in an anthropogenically disturbed, semi-arid estuary

Nutrient budgets in semi-arid estuaries, with ephemeral freshwater inflows and limited nutrient sources, are likely incomplete if contributions from submarine groundwater discharge (SGD) are not included. Here, the relative importance of saline/recirculated SGD-derived nutrient fluxes spatiotemporal variability to the overall nutrient budget is quantified for Nueces Bay, Texas, U.S.A., across hydroclimatic conditions ranging from drought to normal, to flood. On average, 67% of the variance in water quality is due to temporal differences while 16% is explained by spatial differences. Principal component analysis (PCA) reveals three principal components: freshwater inflow (PC1 28.8%), saline/recirculated SGD and recycled nitrogen (PC2 15.6%), and total SGD and "new" nitrogen (PC3 11.2%). Total SGD porewater fluxes ranged from 29.9-690.3 mmol∙m^-2d^-1 for ammonium, 0.21-18.7 mmol∙m^-2d^-1 for nitrite+nitrate, 3.1-51.3 mmol∙m^-2d^-1 for phosphate, 57.1-719.7 mmol∙m^-2d^-1 for silicate, and 95.9-36,838.5 mmol∙m^-2d^-1 for dissolved organic carbon. Total and saline/recirculated SGD fluxes were on average 150-26,000 and 5.8-466 times, respectively, greater than surface runoff fluxes across all seasons. Nitrogen (N) enrichment in porewater occurs near the agricultural fields because of soil N flushing and percolation to groundwater, which facilitates N-rich groundwater fluxes. There were substantial "new" N inputs from terrestrial groundwater following precipitation while saline/recirculated SGD of recycled N accounts for only <4% of total SGD inputs. The "new" N inputs occur in the river and river mouth during flooding, and near the north shore where topography and hydraulic gradients are steeper during drought. Thus, while significant inputs of N may be associated with atmospheric deposition, or remineralization in the porewater, groundwater is the highest contributor to the nutrient budget in Nueces Bay. This result implies that nutrient management strategies should focus on land-use practices to reduce N contamination of shallow groundwater and subsequent contamination of estuaries.

High rates of ammonium recycling in northwestern Lake Taihu and adjacent rivers: An important pathway of nutrient supply in a water column

The ammonium (NH₄⁺) pool in the water column of eutrophic lakes is dynamic and undergoes tightly coupled production and consumption processes because of the metabolism of bacterial and algal communities, particularly in summer. However, NH₄⁺ recycling rates along nutrient gradients at river-lake transitional zones and the extent to which NH₄⁺ regeneration can compensate for consumption have been poorly studied. In August (flood period) and November (normal period), 2016, NH₄⁺ regeneration rates (REGs) and potential uptake rates (U_pots) were measured in northwestern Lake Taihu and adjacent rivers. Results showed that the REGs ranged from 0.09 to 3.30 μmol N L^-1 h^-1 and the U_pots ranged from 0.20 to 4.88 μmol N L^-1 h^-1, with higher recycling rates occurring at the river sites. Yet, the lake sites showed significantly higher water column NH₄⁺ demand (WCAD) than that of the adjacent river sites during both seasons (p < 0.05), probably as a result of the low REGs and the lack of exogenous nitrogen (N) inputs. The flood period showed significantly higher REG and U_pot values than those of the normal period (p < 0.05), probably controlled by higher water temperature and algal biomass. This study confirms that regenerated NH₄⁺ was more important than the ambient NH₄⁺ for sustaining cyanobacterial blooms in northwestern Lake Taihu and indicates that the river-lake transitional zones are key areas for N control in this hypereutrophic system.

Wastewater influences nitrogen dynamics in a coastal catchment during a prolonged drought

Ecosystem function measurements can enhance our understanding of nitrogen (N) delivery in coastal catchments across river and estuary ecosystems. Here, we contrast patterns of N cycling and export in two rivers, one heavily influenced by wastewater treatment plants (WWTP), in a coastal catchment of south Texas. We measured N export from both rivers to the estuary over 2 yr that encompass a severe drought, along with detailed mechanisms of N cycling in river, tidal river, and two estuary sites during prolonged drought. WWTP nutrient inputs stimulated uptake of N, but denitrification resulting in permanent N removal accounted for only a small proportion of total uptake. During drought periods, WWTP N was the primary source of exported N to the estuary, minimizing the influence of episodic storm-derived nutrients from the WWTP-influenced river to the estuary. In the site without WWTP influence, the river exported very little N during drought, so storm-derived nutrient pulses were important for delivering N loads to the estuary. Overall, N is processed from river to estuary, but sustained WWTP-N loads and periodic floods alter the timing of N delivery and N processing. Research that incorporates empirical measurements of N fluxes from river to estuary can inform management needs in the face of multiple anthropogenic stressors such as demand for freshwater and eutrophication.

Mitigating cyanobacterial harmful algal blooms in aquatic ecosystems impacted by climate change and anthropogenic nutrients

Mitigating the global expansion of cyanobacterial harmful blooms (CyanoHABs) is a major challenge facing researchers and resource managers. A variety of traditional (e.g., nutrient load reduction) and experimental (e.g., artificial mixing and flushing, omnivorous fish removal) approaches have been used to reduce bloom occurrences. Managers now face the additional effects of climate change on watershed hydrologic and nutrient loading dynamics, lake and estuary temperature, mixing regime, internal nutrient dynamics, and other factors. Those changes favor CyanoHABs over other phytoplankton and could influence the efficacy of control measures. Virtually all mitigation strategies are influenced by climate changes, which may require setting new nutrient input reduction targets and establishing nutrient-bloom thresholds for impacted waters. Physical-forcing mitigation techniques, such as flushing and artificial mixing, will need adjustments to deal with the ramifications of climate change. Here, we examine the suite of current mitigation strategies and the potential options for adapting and optimizing them in a world facing increasing human population pressure and climate change.