Ambient Nutrient Concentrations Research Articles

Temporal and spatial water quality impacts of point-source versus catchment-derived nitrogen loads in an urbanised estuary

The Brisbane River estuary is an anthropogenically-impacted waterway in southeast Queensland, Australia. The estuary is over 80 km long and flows through an urbanised region. It receives over 500 t per year of total nitrogen (N) from direct point-source discharges in addition to sporadic flood loads of N from an agriculturally impacted upper catchment. Comprehensive water quality monitoring data for the estuary have been collected from at least 2001. This monitoring data includes ambient nutrient concentrations in the estuary, nutrient concentration and volume of the catchment inflows, and nutrient concentration and volume of point source discharges. This long-term data from a range of sources was used to determine temporal and spatial variations in concentrations, forms, stores and loads of N along the estuary for the period 2001 to 2022. Results showed that, during low-flow periods, the store of N in the mid-upper estuary (33–81 km upstream) is significantly determined by point-source discharges to this reach, and therefore the store of N can be modelled. Model parameters are the daily point source loads, a point source load decay factor, and a background constant store. In the lower estuary (0–33 km upstream) N store can be accurately determined based on dilution with seawater, with point sources not having significant influence on total N in the reach. Total N from large flood events was found to largely pass through the estuary without detectable removal processes, delivering catchment derived N directly to coastal waters. This work informs potential application of nutrient offsets in the estuary, guiding where and when offset options will be effective to mitigate the water quality impacts of point-source nutrients.

Freshwater mussels promote functional redundancy in sediment microbial communities under different nutrient regimes

Abstract Animals are a critical component of biogeochemical cycles. While animal mediated fluxes of nutrients and energy have received considerable attention, the impacts of these fluxes on microbial community structure and function are comparatively understudied. Here, we investigated if freshwater mussel influences on biogeochemical cycling in stream sediment are accompanied by changes in sediment microbial community composition and ecoenzymatic activity, and if these relationships change under different nutrient regimes. We predicted that mussel effects on ecosystem function are reflected by modified sediment microbial communities. We hypothesized that if changes in either sediment ecoenzymatic function or microbial community composition are driven by mussel‐derived nutrient amendments, we should see muted changes in microbial community assemblages or function when a given nutrient is abundant. However, if microbial communities and function are influenced by other mussel functions, then we should see uniform changes regardless of nutrient availability. We transplanted freshwater mussels and natural river sediment to flow‐through mesocosms and monitored changes in microbial community composition over 1 week. Our results indicate that mussels always changed sediment microbial community composition, but the way communities changed was dependent on ambient nutrient concentrations. On the final day we measured the activity of ecoenzymes known to correlate to microbial function and nutrient availability. Mussels homogenized the stoichiometric ratios of ecoenzyme activities, indicating a consistent function of sediment microbes associated with freshwater mussels. Our results suggest that mussels may promote functional redundancy in sediment microbial communities and highlight the importance of animals in controlling biogeochemical transformations under changing nutrient conditions. Read the free Plain Language Summary for this article on the Journal blog.

Evaluation of nutrient assimilative capacity in waterfowl impoundments: The role of environmental stressors.

The effectiveness of wetlands in sequestering nutrients and improving water quality relies on a suite of abiotic and biotic conditions. To more fully understand the restraints on nutrient removal, especially salinity and plant cover, we created field-scale mesocosms and monitored nutrient sequestration with nutrient additions and isotopic pool dilutions over 2 years in two wetlands near the Great Salt Lake in Utah. Surprisingly, we found no differences in nutrient removal with plant removal, increased salinity, and altered ambient nutrient concentrations, suggesting functional redundancy in associated primary producers. When submerged aquatic vegetation was removed, chlorophyll α concentration (0.1-9.0μg/L) increased while nitrogen (N) and phosphorus (P) assimilation remained the same as phytoplankton occupied the open niche space. We did find ammonium concentrations to be inversely related to nitrate assimilation-as the ammonium concentration increased, nitrate assimilation decreased, suggesting preferential uptake of ammonium. Last, in our high N and P treatment mesocosms, the nitrate dramatically declined from 43.9mg/L to background levels (<0.1mg/L) within 1 week, showing a high potential for N remediation in these wetlands.

Co-Limitation of Phytoplankton by N and P in a Shallow Coastal Lagoon (Ria Formosa): Implications for Eutrophication Evaluation

AbstractThe Ria Formosa coastal lagoon is a highly productive shallow ecosystem in southern Portugal, subjected to nutrient inputs from anthropogenic and natural sources. Nutrients are major abiotic drivers of phytoplankton in this system, but their effects on phytoplankton assemblages and the occurrence of nutrient limitation are still poorly understood. The main goal of this study was, thus, to evaluate the occurrence, type, and effects of nutrient limitation on phytoplankton community and specific functional groups in the Ria Formosa coastal lagoon. We conducted nutrient enrichment experiments with factorial additions of nitrogen (N) and phosphorus (P) using natural phytoplankton assemblages from distinct locations in the Ria Formosa, throughout a yearly cycle. Phytoplankton composition and abundance were evaluated using inverted and epifluorescence microscopies, and spectrophotometric methods were used for biomass. Limitation was defined as higher phytoplankton growth following enrichment with a particular nutrient in relation to the non-enriched control. The most common type of phytoplankton limitation was simultaneous co-limitation by N and P; diatoms, as r-strategists, were the most frequently limited group. Single N and P limitation, and serial P limitation were also observed, as well as negative responses to nutrient enrichment. Group-specific responses to nutrient enrichment were not reflected in the relative abundance of phytoplankton groups within the whole assemblage, due to the numerical dominance of pico-sized groups (cyanobacteria and eukaryotic picophytoplankton). Ambient nutrient ratios and concentrations did not predict phytoplankton nutrient limitation, given the different nutrient utilisation traits among phytoplankton functional groups. Therefore, nutrient ratios should not be used as indicators of nutrient limitation in eutrophication assessment.

Inputs of seabird guano alter microbial growth, community composition and the phytoplankton-bacterial interactions in a coastal system.

Seabird guano enters coastal waters providing bioavailable substrates for microbial plankton, but their role in marine ecosystem functioning remains poorly understood. Two concentrations of the water soluble fraction (WSF) of gull guano were added to different natural microbial communities collected in surface waters from the Ría de Vigo (NW Spain) in spring, summer, and winter. Samples were incubated with or without antibiotics (to block bacterial activity) to test whether gull guanostimulated phytoplankton and bacterial growth, caused changes in taxonomic composition, and altered phytoplankton-bacteria interactions. Alteromonadales, Sphingobacteriales, Verrucomicrobia and diatoms were generally stimulated by guano. Chlorophyll a (Chl a) concentration and bacterial abundance significantly increased after additions independently of the initial ambient nutrient concentrations. Our study demonstrates, for the first time, that the addition of guano altered the phytoplankton-bacteria interaction index from neutral (i.e. phytoplankton growth was not affected by bacterial activity) to positive (i.e. phytoplankton growth was stimulated by bacterial activity) in the low-nutrient environment occurring in spring. In contrast, when environmental nutrient concentrations were high, the interaction index changed from positive to neutral after guano additions, suggesting the presence of some secondary metabolite in the guano that is needed for phytoplankton growth, which would otherwise be supplied by bacteria.

Effects of filtration timing and pore size on measured nutrient concentrations in environmental water samples

AbstractNutrient monitoring is important for informing management decisions to mitigate eutrophication in aquatic systems. Many nutrient monitoring programs use filter pore sizes that allow microorganisms to pass into samples and/or wait extended times between sample collection and filtration/preservation, allowing microbial processes to alter nutrient concentrations. Here, 34 sites were sampled to determine how filter pore size and filtration timing affected measured ammonium (NH4+) and orthophosphate (ortho‐P) concentrations. Three filter pore sizes (0.22, 0.45, and 0.70 μm) were used to filter water immediately upon collection and after 5 and 22 h in a bottle. NH4+ and ortho‐P concentrations varied relative to “baseline” measurements (i.e., 0.22 μm, field‐filtered samples), both over time and with different filter pore sizes, and showed no predictable direction of change based on ambient nutrient concentration or trophic status. As expected, larger relative changes occurred with lower ambient concentrations; however, for the entire dataset, samples with &gt; 1 μmol L−1 ortho‐P and &gt; 3 μmol L−1 NH4+ were lower by 11 and 33%, respectively, which would result in reported nutrient concentrations that were not representative of in situ conditions. Whole‐water samples filtered after 22 h varied up to 3070% for NH4+ and 480% for ortho‐P from baseline concentrations. Filtering water samples with a 0.22 filter (or 0.45 μm, at worst), immediately upon collection, should be adopted as standard practice to ensure that reported nutrient concentrations represent the most accurate measurement possible. Inconsistent and/or insufficient sampling and sample handling procedures can lead to poorly calibrated models and misinformed management and legislative decisions.

Incorporating parameter variability into Monod models of nutrient-limited growth of non-diazotrophic and diazotrophic cyanobacteria.

Models are widely used tools in aquatic science to understand the mechanism of phytoplankton growth and anticipate the occurrence of harmful algal blooms. However, model parameterization remains challenging and issues that may introduce prediction uncertainty exist. Many models use the Monod equation to predict cyanobacteria growth rate based on ambient nutrient concentrations. The half-saturation concentrations in the Monod equation varies greatly among different studies and depends on environmental conditions. In this study, we estimated the growth rate due to nutrient limitations for two cyanobacteria species (Microcystis aeruginosa and Dolichospermum flos-aquae) using a modified Monod model which allows the half-saturation concentration to vary according to initial nitrogen (N) conditions. The model is calibrated against observations from laboratory experiment where cyanobacteria growth and ambient nutrient concentrations were measured simultaneously, which is rarely done in the literature. Our results show this modified model produce better predictions on growth rate and biomass, indicating many commonly used mechanistic models may need improvement regarding phytoplankton growth representation. Furthermore, our study quantifies the flexibility in cyanobacteria growth parameter across a wide range of environmental N in eutrophic lakes thus provides important information for large-scale modelling applications.

Experimental evaluation of the role of inorganic phosphorus for terrestrial carbon degradation in stream hyporheic zones

Abstract The hyporheic zone (HZ) is a hotspot of carbon processing in stream ecosystems as a consequence of the mixing of organic matter and nutrients from ground water and surface waters. However, major knowledge gaps exist regarding the drivers of microbial activity and carbon processing in the HZ among stream ecosystems with different carbon sources and sediment properties. We investigated the impact of algal dissolved organic matter (DOMalgal) and inorganic phosphorus (P) on the degradation of soil DOM (DOMsoil) by hyporheic microorganisms in two laboratory experiments. In our first experiment, we explored the influence of different ratios of DOMalgal to DOMsoil with and without P additions on microbial respiration and DOM composition in laboratory hyporheic microcosms under oxic conditions. Here, we used glass beads colonised by stream microorganisms resembling a pristine stream system. As the addition of DOMalgal increased P concentrations, we added P to adjust the P concentrations to the same level of the pure DOMalgal in a second batch. In our second experiment, we determined the aerobic microbial respiration of HZ‐sediments from 20 streams along a land‐use gradient in Austria incubated with DOMsoil. Again, we performed the experiments with and without P additions to see whether effects on microbial respiration depended on the ambient P concentrations of the streams. Aerobic microbial respiration in the hyporheic microcosms decreased with increasing DOMsoil proportions. When P concentrations were adjusted to the P level of the DOMalgal, aerobic microbial respiration rates were similar between the different DOM mixtures in the microcosms, mainly stimulating the degradation of humic‐like DOM fractions. This highlights the stimulating effects of the P additions on hyporheic microbial respiration and humic‐like DOM degradation in pristine streams. However, P additions caused a significant increase in microbial respiration in only one of 20 natural HZ‐sediments, suggesting that aerobic microbial respiration rates rarely were controlled by P availability in the investigated streams. We conclude that nutrient pulses can, but do not necessarily, stimulate microbial activity and terrestrial carbon degradation in the HZ of streams. Nevertheless, at low ambient nutrient concentrations (i.e., in pristine streams) terrestrial carbon degradation in the HZ can be accelerated when nutrient pulses occur, which has consequences for CO2 outgassing and the organic matter quality in the stream and its export to downstream sections.

Episodic Nutrient Addition Affects Water Column Nutrient Processing Rates in River‐to‐Lake Transitional Zones

AbstractStorm‐driven nutrient loading from tributaries can fuel eutrophication in nearshore and open water areas of lentic ecosystems. However, nutrient processing in river‐to‐lake transitional zones can substantially alter the amount and composition of nutrients transported to lakes from upstream surface waters. We measured the removal of nutrients and dissolved organic carbon (DOC) from the water column in the Fox rivermouth (Green Bay, Lake Michigan) to evaluate the response of rivermouth plankton to episodic nutrient enrichment. Light and dark water column incubations (8–12 hr) were conducted on four occasions from April through September to measure changes in dissolved nitrogen (N), phosphorus (P), and DOC concentrations in three locations along the Fox rivermouth. Two incubation experiments were conducted on consecutive days, (a) under ambient nutrient concentrations, and (b) under experimentally enriched N and P concentrations. Spatial and temporal variation was observed in nutrient uptake rates, but light incubations consistently had higher nutrient uptake rates than dark incubations. Nutrient enrichment increased total dissolved P and total dissolved N uptake and DOC release in light incubations, but only increased total dissolved P uptake in dark incubations. Moreover, nutrient uptake ratios (N:P) decreased from ambient to nutrient enriched conditions and indicated preferential P uptake by phytoplankton communities in light conditions. Our study substantiates that rivermouths can process nutrients bound for downstream ecosystems and demonstrates the potential of plankton communities to dynamically increase net uptake rates in response to episodic nutrient enrichment.

Characterizing ambient nutrient concentrations and potential warning levels for surface water in natural forested wetlands in the Piedmont and Coastal Plain of North Carolina, USA

Nearly thirty years ago the U.S. EPA directed states to begin developing water quality standards specific to wetlands; however, progress has been limited. This study presents an overview of ambient wetland water quality and the first step towards the development of numeric nutrient standards for forested wetlands in North Carolina. Water quality samples were collected in 16 natural wetlands across the state for three years. The sites span a range of anthropogenic disturbance. The data was combined with historical data collected at these and other sites from 2005 to 2013. Concentration levels in wetlands without significant anthropogenic disturbance are likely higher than streams for TN, TP, similar for NH4+, and likely lower than streams for NO3–-N. A method developed by the U.S. EPA to establish reference levels was modified to define draft nutrient warning levels that may indicate anthropogenic impacts. These warning levels were calculated as 0.06 mg/L, 0.14 mg/L, 0.34 mg/L and 2.8 mg/L for NO3–-N, NH4+-N, TP and TN, respectively. NO3–-N and NH4+-N concentrations were the best indicators of disturbance. Overall, despite the limitations imposed by the sampling frequency and inherent variability in wetland ecosystems, this research methodology presents a reasonable first attempt at defining nutrient warning levels for undisturbed natural forested wetlands in N.C., and may serve as a starting point in developing numeric nutrient criteria for wetlands.

Trichome Lengths of the Heterocystous N2-Fixing Cyanobacteria in the Tropical Marginal Seas of the Western North Pacific

Calothrix rhizosoleniae and Richelia intracellularis are heterocystous cyanobacteria found in the tropical oceans. C. rhizosoleniae commonly live epiphytically on diatom genera Chaetoceros (C-C) and Bacteriastrum (B-C) while R. intracellularis live endosymbiotically within Rhizosolenia (R-R), Guinardia (G-R), and Hemiaulus (H-R); although, they occasionally live freely (FL-C and FL-R). Both species have much shorter trichomes than the other marine filamentous cyanobacteria such as Trichodesmium spp. and Anabaena gerdii. We investigated the trichome lengths of C. rhizosoleniae and R. intracellularis in the South China Sea (SCS) and the Philippine Sea (PS) between 2006 and 2014. On average, H-R had the shortest trichome lengths (3.5 cells/trichome), followed by B-C and C-C (4.9–5.2 cells/trichome) and FL-C (5.9 cells/trichome), and R-R, G-R, and FL-R had the longest trichome lengths (7.4–8.3 cells/trichome). Field results showed the trichome lengths of C-C and B-C did not vary seasonally or regionally. However, FL-C and H-R from the SCS and during the cool season had longer trichomes, where/when the ambient nutrient concentrations were higher. R-R, G-R, and FL-R also showed regional and seasonal variations in trichome length. Ultrastructural analysis found no gas vesicles within the C. rhizosoleniae cells to assist in buoyancy regulation. Results suggest that the trichome lengths of C. rhizosoleniae and R. intracellularis might be regulated by their diatom hosts’ symbiotic styles and by ambient nutrients. Short trichome length might help C. rhizosoleniae and R. intracellularis to stay in the euphotic zone regardless as to whether they are free-living or symbiotic.

Combined carbon flows through detritus, microbes, and animals in reference and experimentally enriched stream ecosystems.

Tracking carbon (C) flow through ecosystems requires quantification of myriad biophysical processes, including C routing through microbial and metazoan food webs. Yet detailed organic matter budgets are rarely combined with simultaneous measurement of C flows supporting microbial and animal production. Here, we synthesize concurrent data sets on organic matter, microbes, and macroinvertebrates from two detritus-based stream ecosystems, one of which was subject to experimental nitrogen (N) and phosphorus (P) enrichment. Our synthesis provides new insights into C flow through forest stream ecosystems. Over 3 yr, the reference stream showed a striking balance of inputs and outputs, with a mean surplus of only 7g C·m-2 ·yr-1 (~1% of annual inputs), presumably stored in sediments as fine particulate organic matter (FPOM). In contrast, N and P enrichment over 2 yr resulted in severe deficits of C (-576g C·m-2 ·yr-1 or~170% of annual inputs), a shortfall presumably met by stored C. Our data set provides an ecosystem-based estimate of the fate of forest litter C at ambient nutrient concentrations: 6.2% was leached as dissolved organic C, 40.6% and 8.5% flowed to litter-associated fungi and bacteria, respectively, 7.5% was consumed by macroinvertebrates, 1.8% was exported as coarse particles, and the remainder (35.4%) was presumably fragmented by biophysical processes. Our calculations also allowed an estimate of inputs into the heterogeneous FPOM pool, which is otherwise difficult to obtain. At naturally low nutrient concentrations, 50.7% was derived from fragmented litter, 39.1% from microbial biomass (mostly fungal), and 10.2% from macroinvertebrate egesta. Nutrient addition drove large changes in C fluxes in the experimental stream, especially in flows of leaf litter to fungi (×1.7 pretreatment) and macroinvertebrates (×2.7), and of FPOM to hydrologic export (×2.6). Our results underscore the key roles of both microbes and metazoans in controlling C flow through detritus-based ecosystems, as well as how release from persistent nutrient limitation may perturb steady-state conditions of C inputs vs. outputs. Our analysis also suggests areas for future research, including assessing the relative importance of stored vs. recycled C in fueling detrital food webs subject to altered nutrient regimes and other global-change drivers.

Nutrient-imbalanced conditions shift the interplay between zooplankton and gut microbiota

BackgroundNutrient stoichiometry of phytoplankton frequently changes with aquatic ambient nutrient concentrations, which is mainly influenced by anthropogenic water treatment and the ecosystem dynamics. Consequently, the stoichiometry of phytoplankton can markedly alter the metabolism and growth of zooplankton. However, the effects of nutrient-imbalanced prey on the interplay between zooplankton and their gut microbiota remain unknown. Using metatranscriptome, a 16 s rRNA amplicon-based neutral community model (NCM) and experimental validation, we investigated the interactions between Daphnia magna and its gut microbiota in a nutrient-imbalanced algal diet.ResultsOur results showed that in nutrient-depleted water, the nutrient-enriched zooplankton gut stimulated the accumulation of microbial polyphosphate in fecal pellets under phosphorus limitation and the microbial assimilation of ammonia under nitrogen limitation. Compared with the nutrient replete group, both N and P limitation markedly promoted the gene expression of the gut microbiome for organic matter degradation but repressed that for anaerobic metabolisms. In the nutrient limited diet, the gut microbial community exhibited a higher fit to NCM (R2 = 0.624 and 0.781, for N- and P-limitation, respectively) when compared with the Control group (R2 = 0.542), suggesting increased ambient-gut exchange process favored by compensatory feeding. Further, an additional axenic grazing experiment revealed that the growth of D. magna can still benefit from gut microbiota under a nutrient-imbalanced diet.ConclusionsTogether, these results demonstrated that under a nutrient-imbalanced diet, the microbes not only benefit themselves by absorbing excess nutrients inside the zooplankton gut but also help zooplankton to survive during nutrient limitation.

Evidence of Phytoplankton Light Acclimation to Periodic Turbulent Mixing Along a Tidally Dominated Tropical Coastline

AbstractOne of the largest tropical tidal ranges in the world occurs in King Sound, a semienclosed embayment in the tropical Kimberley region of Western Australia. Incubations of phytoplankton within King Sound displayed reduced photosynthetic efficiency, elevated maximum photosynthetic rates, and no measurable photoinhibition. A response typical of high light adapted phytoplankton despite decreased water clarity and low ambient nutrient concentrations in the estuary. This is in contrast with the adjacent shelf where phytoplankton, associated with a deep chlorophyll maximum, display high photosynthetic efficiency and strong light inhibition typical of low light adaptation. Remote sensing and numerical modeling suggest that spatial and temporal variations in tidal mixing drive changes in light variability and in photoacclimation. In King Sound phytoplankton experience the largest variations in light over short time scales where diatoms dominate since they can rapidly acclimate to water column light conditions by adjusting pigment within the cell. The photophysiological response of the phytoplankton in the Sound suggests that acclimation to alternate weak and strong mixing exposes them to cyclical changes in light intensity delaying the onset of photoinhibition, allowing higher maximum photosynthetic rates to be attained. These findings highlight the importance of a multifaceted approach to understanding the links between physics and photoacclimation strategies employed by phytoplankton to more accurately determine rates of depth‐integrated productivity in complex coastal areas.

The influence of Arctic Fe and Atlantic fixed N on summertime primary production in Fram Strait, North Greenland Sea

Climate change has led to a ~ 40% reduction in summer Arctic sea-ice cover extent since the 1970s. Resultant increases in light availability may enhance phytoplankton production. Direct evidence for factors currently constraining summertime phytoplankton growth in the Arctic region is however lacking. GEOTRACES cruise GN05 conducted a Fram Strait transect from Svalbard to the NE Greenland Shelf in summer 2016, sampling for bioessential trace metals (Fe, Co, Zn, Mn) and macronutrients (N, Si, P) at ~ 79°N. Five bioassay experiments were conducted to establish phytoplankton responses to additions of Fe, N, Fe + N and volcanic dust. Ambient nutrient concentrations suggested N and Fe were deficient in surface seawater relative to typical phytoplankton requirements. A west-to-east trend in the relative deficiency of N and Fe was apparent, with N becoming more deficient towards Greenland and Fe more deficient towards Svalbard. This aligned with phytoplankton responses in bioassay experiments, which showed greatest chlorophyll-a increases in + N treatment near Greenland and + N + Fe near Svalbard. Collectively these results suggest primary N limitation of phytoplankton growth throughout the study region, with conditions potentially approaching secondary Fe limitation in the eastern Fram Strait. We suggest that the supply of Atlantic-derived N and Arctic-derived Fe exerts a strong control on summertime nutrient stoichiometry and resultant limitation patterns across the Fram Strait region.

Identifying diatom indicator species of nutrient enrichment: An in situ nutrient enrichment experiment in subtropical upland streams

Nutrients are a fundamental determinant of broad scale patterns in the benthic algal community of stream ecosystems. Excessive inputs of nutrients such as nitrogen and phosphorus into upland streams may result in significant impacts on benthic algal communities. In upland streams, diatoms are the most abundant and diverse taxonomic group in benthic algal communities. In this study, benthic algal biomass and diatom community responses to in situ nitrogen and phosphorus enrichment were investigated by conducting an in-stream nutrient diffusing substrate experiment at high and low ambient nutrient concentration sites. Nitrogen was found to be the primary limiting nutrient across all sites, regardless of the ambient nutrient concentrations. In general, enrichment relieved nutrient limitation, promoting the dominance of a smaller suite of eutrophic species in the community. This was particularly evident in the low ambient nutrient sites where oligotrophic species initially dominated the community. This study identified diatom species with competitive advantage under nutrient enriched conditions and further the understanding of diatom species competition for multiple nutrients. Five diatom nutrient trait groups differentiate nutrient generalists and specialists and identifies potential indicator species based on their responses to specific enrichment treatments.

Physiological and Ecological Responses of Photosynthetic Processes to Oceanic Properties and Phytoplankton Communities in the Oligotrophic Western Pacific Ocean.

Understanding the dynamics of primary productivity in a rapidly changing marine environment requires mechanistic insight into the photosynthetic processes (light absorption characteristics and electron transport) in response to the variability of environmental conditions and algal species. Here, we examined the photosynthetic performance and related physiological and ecological responses to oceanic properties [temperature, salinity, light, size-fractionated chlorophyll a (Chl a) and nutrients] and phytoplankton communities in the oligotrophic Western Pacific Ocean (WPO). Our results revealed high variability in the maximum (Fv/Fm; 0.08–0.26) and effective (Fq′/Fm′; 0.02–0.22) photochemical efficiency, the efficiency of charge separation (Fq′/Fv′; 0.19–1.06), the photosynthetic electron transfer rates (ETRRCII; 0.02–5.89 mol e– mol RCII–1 s–1) and the maximum of primary production [PPmax; 0.04–8.59 mg C (mg chl a)–1 h–1]. All these photosynthetic characteristics showed a depth-specific dependency based on respective nonlinear regression models. On physiological scales, variability in light absorption parameters Fv/Fm and Fq′/Fm′ notably correlated with light availability and size-fractionated Chl a, while both ETRRCII and PPmax were correlated to temperature, light, and ambient nutrient concentration. Since the presence of nonphotochemical quenching (NPQNSV; 2.33–12.31) and increasing reductant are used for functions other than carbon fixation, we observed nonparallel changes in the ETRRCII and Fv/Fm, Fq′/Fm′, Fq′/Fv′. In addition, we found that the important biotic variables influencing Fv/Fm were diatoms (cells > 2 μm), picosized Prochlorococcus, and eukaryotes, but the PPmax was closely related to large cyanobacteria (cells > 2 μm), dinoflagellates, and picosized Synechococcus. The implication is that, on ecological scales, an interaction among temperature, light, and nutrient availability may be key in driving the dynamics of primary productivity in the WPO, while large cyanobacteria, dinoflagellates, and picosized Synechococcus may have a high contribution to the primary production. Overall, the photosynthetic processes are interactively affected by complex abiotic and biotic variables in marine ecosystems, rather than by a single variable.

Seasonal Nitrogen Uptake Dynamics and Harmful Algal Blooms in the York River, Virginia

During a 2-year study of planktonic nitrogen (N) nutrition, temporal variability of (1) ambient nutrient concentrations; (2) uptake rates of ammonium (NH4+), nitrate (NO3−), nitrite (NO2−), urea, and amino acids (AA) in three size fractions (> GF/F, > 5 μm, and 5–0.2 μm); (3) NH4+ regeneration and NO3− regeneration (nitrification); and (4) an unexpected bloom of Alexandrium monilatum were examined. Dissolved organic N (DON) was the most abundant form of fixed N. High concentrations of NH4+ and NO2− were detected during the late summer and fall, reaching maximums of 9.9 and 7.6 μmol N L−1, respectively. The highest uptake rates were for NH4+ at all stations, size fractions, and seasons sampled and ranged from 34 to 80% of total absolute N uptake. The magnitude of uptake rates in the > GF/F fraction generally followed the pattern of NH4+ > NO3− > urea > AA > NO2− with some exceptions when urea uptake rates were higher than NO3−. Rates of NH4+ regeneration and nitrification often exceeded uptake rates, indicating autochthonous pathways for nutrient loading. Exceptionally high dinoflagellate biomass was found in late summer and corresponded with harmful algal blooms. Kinetic curves measured during an A. monilatum bloom showed high Vmax (33.7 ± 2.7 × 10−3 h−1) and high Ks (7.3 μmol N L−1) for NH4+ suggesting that it can rapidly utilize high concentrations when available but may be outcompeted by other phytoplankton when concentrations of NH4+ are low. However, A. monilatum demonstrated that it is capable of using a diverse suite of N substrates, giving it a potential competitive advantage under diverse nutrient conditions.

Low growth resilience of subarctic rhodoliths (Lithothamnion glaciale) to coastal eutrophication

Eutrophication is one of the most important drivers of change in coastal marine ecosystems worldwide. Given their slow growth, rhodoliths and the biodiverse communities they support are regarded as non-renewable resources threatened by human activity. Consequences of nutrient enrichment on growth and calcification in crustose coralline algae are equivocal, and even more so in cold-water rhodoliths. We paired a 183 d laboratory mesocosm experiment with a 193 d field experiment on Newfoundland (Canada) rhodoliths (Lithothamnion glaciale) to test the hypothesis that nutrient (nitrate, ammonia, and phosphate) enrichment and biofouling reduce rhodolith growth. Rhodoliths in the laboratory were exposed to 1 of 3 nutrient concentrations (ambient, intermediate, or high) and either of 2 levels of manual cleaning (cleaned or uncleaned) to control biofouling. We exposed rhodoliths in the field to 1 of 2 nutrient concentrations (ambient or elevated). Eutrophication in the laboratory did not affect biofouling; however, manual cleaning reduced biofouling by ~4 times relative to uncleaned rhodoliths. Rhodoliths grew 2 times slower at elevated than ambient nutrient concentrations, and ~27% more in cleaned than uncleaned rhodoliths at all concentrations. Rhodoliths in the field also grew significantly slower under elevated than ambient phosphate concentrations, but only during the first 6 wk, indicating some capacity for long-term recovery. We conclude that despite some growth resilience to low and infrequent increases in nutrient concentrations, subarctic L. glaciale rhodoliths cannot cope with prolonged exposure to modest eutrophication.

Nutrition of the seagrass Cymodocea nodosa: Pulses of ammonium but not of phosphate are crucial to sustain the species growth

We investigated the nutrient acquisition of the seagrass Cymodocea nodosa and the importance of sediment to water column nutrient pulses, which regularly occur in coastal lagoons forced by incoming flood tides. Ammonium was preferentially taken up and accounted for 60–90% of the total daily N acquisition, whereas amino acid acquisition through belowground plant parts represented the second most important source of N, accounting 8–34%. The uptake of ammonium pulses increased dramatically the daily N acquisition from 9.5 mmol m−2 day−1 to 1.33 mmol m−2 day−1 at ambient nutrient concentrations, enabling the species to meet its N demand. Roots accounted for 96% of the total phosphate acquisition, with no evidence for P limitation. The ability of seagrasses to adapt to nutrient pulses may be an overlooked nutrition strategy common to many ecosystems where nutrient availability in the water column is low but nutrient pulses occur.