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Harmful cyanobacterial blooms (CyanoHABs) are linked to increasing anthropogenic nitrogen (N) and phosphorus (P) inputs. However, CyanoHABs in many large lakes continue despite extensive abatement efforts, mostly focused on external P loading. Internal nutrient cycling can modify nutrient availability and limitation; thus, understanding the relative importance of external vs. internal nutrient loading is essential for developing effective mitigation strategies for CyanoHABs. We estimated long‐term nutrient budgets for Lake Taihu, China, from mass balance models using extensive monitoring of input and output nutrient data from 2005 to 2018 to quantify contributions from internal nutrient loading. The nutrient mass balance showed that 9% and 63% of annual external N and P inputs, respectively, were retained in the lake. Denitrification removed 54% of external N loading and can thus help explain rapid decreases in lake N concentrations and summer N limitation. Water column NH4+regeneration can help sustain CyanoHABs over the short term and contributed 38–58% of potential NH4+demand for summer‐fall, Microcystis‐dominated blooms. Internal P release contributed 23–90% of CyanoHABs P demand, although Taihu was a net P sink on an annual scale. Our results show that internal nutrient cycling helps sustain CyanoHABs in Taihu, despite reductions in external nutrient inputs. Furthermore, N is leaving the lake faster than P, thereby creating persistent N limitation. Therefore, parallel reductions in external N loading, along with P, will be most effective in reducing CyanoHABs and accelerate the recovery process in this and other large, shallow lakes.

Eutrophication mitigation is an ongoing priority for aquatic ecosystems. However, the current eutrophication control strategies (phosphorus (P) and/or nitrogen (N)) are guided mainly by nutrient addition experiments in small waters without encompassing all in-lake biogeochemical processes that are associated largely with lake morphological characteristics. Here, we use a global lake data set (573 lakes) to show that the relative roles of N vs P in affecting eutrophication are underpinned by water depth. Mean depth and maximum depth relative to mixing depth were used to distinguish shallow (mixing depth > maximum depth), deep (mixing depth < mean depth), and transitional (mean depth ≤ mixing depth ≤ maximum depth) lakes in this study. TN/TP ratio (by mass) was used as an indicator of potential lake nutrient limitation, i.e., N only limitation if N/P < 9, N + P colimitation if 9 ≤ N/P < 22.6, and P only limitation if N/P ≥ 22.6. The results show that eutrophication is favored in shallow lakes, frequently (66.2%) with N limitation while P limitation predominated (94.4%) in most lakes but especially in deep ones. The importance of N limitation increases but P limitation decreases with lake trophic status while N and P colimitation occurs primarily (59.4%) in eutrophic lakes. These results demonstrate that phosphorus reduction can mitigate eutrophication in most large lakes but a dual N and P reduction may be needed in eutrophic lakes, especially in shallow ones (or bays). Our analysis helps clarify the long debate over whether N, P, or both control primary production. While these results imply that more resources be invested in nitrogen management, given the high costs of nitrogen pollution reduction, more comprehensive results from carefully designed experiments at different scales are needed to further verify this modification of the existing eutrophication mitigation paradigm.

Deciphering molecular structures of dissolved organic matter (DOM) components is key to understanding the formation and transformation of this globally important carbon pool in aquatic environments. Such a task depends on the integrated use of complementary analytical techniques. We characterize the molecular structure of natural DOM using an ion mobility quadrupole time of flight liquid chromatography mass spectrometer (IM Q-TOF LC/MS), which provides multidimensional structural information on DOM molecules. Geometric conformation of DOM molecules is introduced into molecular-level analysis via the ion mobility (IM) in the system, and an actual measurement of isomers is achieved for the first time. Our data show that natural DOM molecules from several south Texas rivers and adjacent coastal waters have smaller geometric conformation compared with standard biomolecules. Furthermore, about 10% of all DOM molecules resolved within the detection limit of IM-MS had at least one but no more than four isomers. With acquired geometric and isomeric information, we established a multidimensional database containing 89 natural DOM compounds. This database provides a foundation to expand further, or compare, with DOM data from different seasons and locations.

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.

Remineralization of labile organic nitrogen is a key part of nitrogen cycling in seawater, yet factors controlling this process remain poorly understood. The degradation and fate of amino acids in coastal waters were investigated with incubation experiments using 15N‐labeled alanine (Ala). The cycling of Ala, including the release of NH4+through remineralization, the subsequent removal of NH4+via assimilation and nitrification, and the formation of refractory dissolved organic nitrogen (DON), was more rapid in subsurface (mid‐depth 8 m and bottom 17 m) than in surface coastal waters (2 m) of the northern Gulf of Mexico in summer. This pattern is attributed to the different levels of soluble reactive phosphorus (SRP) observed at different depths of the water column. The addition of orthophosphate to the surface water with low SRP (<0.1 μmol L−1) significantly accelerated the remineralization of amino acid, nitrification, and the formation of refractory DON. The rapid remineralization of amino acid and further nitrification in SRP‐replete waters are linked to the rapid growth of heterotrophic bacteria and ammonia‐oxidizing archaea, respectively. Taken together, these results suggest that subsurface coastal waters enriched with SRP are pre‐primed for rapid organic N metabolisms, providing mechanistic insights into nitrogen cycling in marine waters. In this study, we investigated the effect of phosphorus in the degradation of labile organic nitrogen in Gulf of Mexico waters. By using nitrogen stable isotope (15N) labeled amino acid, we were able to trace the fate of amino acid nitrogen at different depths of the water column. We found that subsurface water with a phosphorus concentration higher than 0.5 μmol L−1had a more rapid decomposition rate of amino acid, with faster transformation of amino acid nitrogen to nitrate, and increased formation of relatively refractory dissolved organic nitrogen. This stimulating effect of phosphorus in subsurface water is achieved through interactions between phosphorus and the microbial community. Specifically, high concentrations of phosphorus (>0.5 μmol L−1) promote the development of ammonia‐oxidizing archaea, and therefore accelerate the transformation to nitrate. In addition, high phosphorus concentrations also enhance the growth of heterotrophic bacteria, which can convert labile amino acid to refractory bacteria‐derived organic nitrogen. Taken together, these results suggest that subsurface coastal waters, due to the high phosphorus level, are pre‐primed for rapid organic nitrogen metabolisms. Our study provides mechanistic insights into nitrogen cycling in surface and subsurface marine waters. A high P level (≥0.5 μmol L−1) stimulates the degradation of labile organic nitrogenThe accelerated nitrification rates under high P level are attributed to the development of ammonia‐oxidizing archaeaThe formation of refractory dissolved organic nitrogen is related to fast‐growing heterotrophic bacteria A high P level (≥0.5 μmol L−1) stimulates the degradation of labile organic nitrogen The accelerated nitrification rates under high P level are attributed to the development of ammonia‐oxidizing archaea The formation of refractory dissolved organic nitrogen is related to fast‐growing heterotrophic bacteria

We conducted continuous‐flow experiments on intact sediment cores from Laguna Madre, Sabine Lake, East Matagorda Bay, and Nueces Estuary to evaluate internal nitrogen (N) sources, sinks, and retention mechanisms in Texas estuaries having different salinities. Mean ammonium (NH4+) flux ranged from slight uptake (negative values) to NH4+production rates of about 300 µmol m−2h−1(units used for all N rates) and increased with salinity (p = 0.10). Net nitrate (NO3−) flux (–20 to 32) and net N2flux (–70 to 100) did not relate to salinity. Mean net N2flux was positive but near zero, indicating that N2sources and sinks are nearly balanced. Total denitrification, N fixation, and potential dissimilatory NO3−reduction to NH4+(DNRA) rates were estimated after inflow water was enriched with 15NO3−(100 µmol L−1). Total denitrification rates ranged from 0 to 90 versus N fixation rates ranging from 0 to 97. Potential DNRA, measured conservatively as 15H accumulation, ranged from 0 to 80 and related significantly to salinity (p < 0.01). Increases in total NH4+release after 15NO3−additions were higher but closely related (r = 0.9998) to 15NH4+accumulation, implying exchange reactions of DNRA‐regenerated 15N4+with sediment‐bound 14NH4+. The fate of NO2 was related to salinity, perhaps via sulfide effects on DNRA. Potential DNRA was high in southeastern Corpus Christi Bay in August during hypoxia when the sulfide transition zone was near the sediment surface. Nitrogen fixation and DNRA are important mechanisms that add and retain available N in Texas estuaries.

Isotope‐dilution experiments with 15NH4+were conducted on near‐surface water samples in the Mississippi River plume during May 1992 and July 1993 to quantify community cycling rates for ammnonium and determine whether regeneration rates are enhanced by light. Experiments done under natural light in May showed ranges of potential uptake and regeneration rates of 0–0.4 µM h−1and 0–0.18 µM h−1. Samples collected offshore from the Atchafalaya River and the Southwest Pass of the Mississippi River in July yielded potential uptake rates of 0.4–1.8 µM h−1under natural light vs. 0–0.45 µM h−1in the dark. Ammonium regeneration rates ranged from 0.08 to 0.75 µM h−1in the light and from 0.02 to 0.3 µM h−1in the dark. The observed light/dark regeneration‐rate differences imply a close coupling between phytoplankton production and ammonium regeneration. The ratio of bacterial cell‐specific regeneration to uptake rates increased in the outer regions of the plume, indicating a changing contribution of bacterial‐sized organisms to nitrogen cycling processes in different regions of the plume.

Plankton ecology was examined during the 1986 winter in Grand Traverse Bay, a 190 m deep, fjordlike bay on Lake Michigan. Before ice cover, algal concentration was low and uniformly distributed with depth, as it is in open Lake Michigan. During ice cover (February and March), a bloom of a typical winter-spring phytoplankton community developed in the upper 40 m, resulting in a 4 to 7-fold increase in feeding rate of adult Diaptomus spp. High algal concentration and zooplankton feeding persisted after ice melt (April). During and after ice cover, lipid concentrations of Diaptomus dropped rapidly from 34% of dry weight to 17 % because of egg production. High incident photosynthetically active radiation (PAR), high (45–50%) PAR transmittance of the ice due to little snow on the ice, and water column stability were probably responsible for the bloom. High ice transparency may be a common feature of large lakes and bays, where strong winds blow snow cover off the ice, or at low latitudes where snowmelt due to occasional rains and warm temperature is common. Winter reproducing calanoid copepods use these blooms to increase their reproductive output.

The effects of wheat streak mosaic virus (WSMV) were evaluated on 9 spring wheat (Triticum aestivumL.) cultivars in 1969, 14 spring and 24 winter wheat cultivars in 1970, and 21 winter wheat cultivars in 1971. Plants were mechanically inoculated and the effects measured relative to control plants. Seed yield, plant height, test weight, 1000‐seed weight, and seeds per head were drastically reduced by the virus, whereas percent protein increased. In some instances heads per tiller and plants per plot were decreased slightly. Spring wheat cultivars ‘Chris,’ ‘Bonanza,’ and ‘Fortuna’ and winter wheat cultivars ‘Scout,’ ‘Scout 66,’ ‘Lancer,’ ‘Bronze,’ ‘SD6716,’ ‘SD66171,’ ‘OK6955033,’ ‘NB68508,’ ‘SD6742‐1,’ and ‘Trader’ were found to possess desirable degrees of tolerance to WSMV as measured by both yield of inoculated plants and percent yield reduction as a result of infection. Tolerance shown by these cultivars can make them economically important in areas where the disease is pandemic and useful in breeding programs.

This article describes a sampling device for the chemical analysis and evaluation of a water supply that is constructed by connecting a synchronous electric motor to an overflow pipet and samples the water at periodic intervals. With a motor that rotates at an appropriate rate and a pipet that delivers an appropriate volume, this sampling device delivers a composite sample of desired volume over a given period. The sample comes into contact with only glass or Teflon parts of the device.

A breeding line of spring wheat, C.I. 15092, selected from Triticum aestivumL. em Thell./Agropyron intermedium(Host) Beauv. bred true for immunity from wheat streak mosaic virus (WSMV). F1hybrids of C.I. 15092/T. aestivum‘Fletcher’ had 20 paired and 2 unpaired chromosomes of unequal size at metaphase I of meiosis in pollen mother cells. Apparently C.I. 15092 had one Agropyronchromosome pair carrying genes for immunity from WSMV substituted for a homoeologous wheat chromosome pair. Immunity from WSMV was completely dominant in hybrids. Inheritance data indicated that the Agropyronchromosome was transmitted through 27% male and 21% female gametes of the F1hybrid C.I. 15092/Fletcher. A small number of immune F2and F3plants were comparable to control plants in seed set, indicating that the whole Agropyronchromosome added to the wheat complement was not strikingly harmful in some plants, at least, not to self‐fertility.