Water Column Metabolism Research Articles

Subsidy-stress responses of ecosystem functions along experimental freshwater salinity gradients

Human activity is increasing salt concentrations in freshwaters worldwide, but effects of freshwater salinity gradients on biogeochemical cycling are less understood than in saline, brackish, or marine environments. Using controlled microcosm experiments, we characterized (1) short-term (one to five days) biogeochemical responses and (2) water column metabolism along a freshwater salinity gradient of multiple salt types. After one day, microcosms were oxic (4.48–7.40 mg O2 L−1) but became hypoxic (1.20–3.31 mg L−1) by day five. After one day in oxic conditions, microbial respiration in magnesium-, sodium-, and sea salt-based salinity treatments showed a subsidy-stress response, with respiration increasing by over 100% as salinity increased from 30 to 350–800 µS cm−1. Conversely, respiration consistently increased along a calcium-based salinity gradient, peaking at 1500 µS cm−1. By day five, an inverse subsidy-stress response was observed with elevated respiration at upper or lower ends of the gradient except for the magnesium treatment, which had the lowest respiration at the highest salinity. Calcium- and magnesium-based salinity treatments also caused considerable changes in phosphorus concentrations and C:P and N:P. In a separate experiment, microbial respiration and water column primary production also displayed subsidy-stress responses, but imbalances in effect sizes caused consistently declining net community production with increasing salinity. Collectively, our results establish that short-term exposure to different salt ion concentrations can enhance freshwater biogeochemical cycling at relatively low concentrations and alter resource stoichiometry. Furthermore, the nature of effects of freshwater salinization may also change with oxygen availability.

Patterns and Regulation of Hypolimnetic CO2 and CH4 in a Tropical Reservoir Using a Process‐Based Modeling Approach

AbstractHypolimnetic waters and sediments of lakes and reservoirs are active sites of carbon dioxide (CO2) and methane (CH4) production and accumulation. These potent carbon greenhouse gases (C‐GHG) can be emitted to the atmosphere during water column mixing and, for hydropower reservoirs, through deep water discharge from the outflow. Deep CO2 and CH4 concentrations are regulated by sediment‐water flux, aerobic respiration, methanogenesis, CH4 oxidation, and vertical diffusion. These processes are interlinked but have mostly been studied individually, making their relative importance and interactions difficult to disentangle. In this study, we combined empirical observations with an integrative process‐based modeling to examine ecosystem‐scale dynamics of CO2 and CH4 in the deep layer of a permanently stratified reservoir located on the tropical Borneo Island (Malaysia). Oxygen (O2) was a key modulator of deep CO2 and CH4 metabolism, and shaped their spatial and temporal patterns within the reservoir. Our model reproduced the shape of the vertical profiles of gas concentration and isotopic signature, and suggested that deep C‐GHG stocks reached a steady‐state about 10 years after impoundment. Vertical gas diffusivity was the most determinant parameter controlling reservoir deep C‐GHG accumulation through its direct effect on gas transport and its interaction with water column metabolism through O2 regulation. Model results highlighted the interactions, relative importance, and uncertainties of processes controlling deep C‐GHG fluxes. This case study offers an in‐depth comprehension of C‐GHG storage and emissions in a tropical system, as well as a mechanistic framework transferable to other lakes and reservoirs.

Water column contributions to the metabolism and nutrient dynamics of mid-sized rivers

Lotic and lentic ecosystems are traditionally viewed as dominated by either benthic or water column processes. However, mid-sized rivers represent a transition zone where both benthic and water column processes may both contribute substantially to ecosystem dynamics. Ecosystem processes such as gross primary production (GPP), ecosystem respiration (ER), or nutrient uptake, and the relative contribution of the water column to these processes at the reach scale, are poorly understood in non-wadeable, mid-sized rivers. To clarify the role of the water column at the reach-scale, and to quantify controls on water column processes, we measured GPP, ER, and uptake of nitrate (NO3−), ammonium (NH4+), and soluble reactive phosphorus (SRP) in the water columns of 15 mid-sized rivers (discharge: 13.5–83.3 m3 s−1) spanning nutrient and total suspended solids gradients. We compared water column metabolic and nutrient uptake rates to reach-scale rates to estimate the contribution of the water column to the entire river. Water column metabolism was autotrophic on the day when measured, GPP increased with nutrient availability, and the water column contributed more to whole river GPP than to ER. Water column nutrient uptake increased with GPP across solutes, and there was a positive relationship between human land use and water column uptake of NO3−–N and SRP. The water column accounted for a substantial proportion of reach-scale metabolism and nutrient uptake, but this contribution depended on suspended material and nutrient availability. Integrating the water column into theory describing lotic ecosystem function should clarify mechanisms controlling metabolism and nutrient processing and enhance management of non-wadeable rivers.

Changing sources and processes sustaining surface CO<sub>2</sub> and CH<sub>4</sub> fluxes along a tropical river to reservoir system

Abstract. Freshwaters are important emitters of carbon dioxide (CO2) and methane (CH4), two potent greenhouse gases (GHGs). While aquatic surface GHG fluxes have been extensively measured, there is much less information about their underlying sources. In lakes and reservoirs, surface GHG can originate from horizontal riverine flow, the hypolimnion, littoral sediments, and water column metabolism. These sources are generally studied separately, leading to a fragmented assessment of their relative role in sustaining CO2 and CH4 surface fluxes. In this study, we quantified sources and sinks of CO2 and CH4 in the epilimnion along a hydrological continuum in a permanently stratified tropical reservoir (Borneo). Results showed that horizontal inputs are an important source of both CO2 and CH4 (>90 % of surface emissions) in the upstream reservoir branches. However, this contribution fades along the hydrological continuum, becoming negligible in the main basin of the reservoir, where CO2 and CH4 are uncoupled and driven by different processes. In the main basin, vertical CO2 inputs and sediment CH4 inputs contributed to on average 60 % and 23 % respectively to the surface fluxes of the corresponding gas. Water column metabolism exhibited wide amplitude and range for both gases, making it a highly variable component, but with a large potential to influence surface GHG budgets in either direction. Overall our results show that sources sustaining surface CO2 and CH4 fluxes vary spatially and between the two gases, with internal metabolism acting as a fluctuating but key modulator. However, this study also highlights challenges and knowledge gaps related to estimating ecosystem-scale CO2 and CH4 metabolism, which hinder aquatic GHG flux predictions.

The varying role of water column nutrient uptake along river continua in contrasting landscapes

Nutrient transformation processes such as assimilation, dissimilatory transformation, and sorption to sediments are prevalent in benthic zones of headwater streams, but may also occur in the water column. The river continuum concept (RCC) predicts that water column processes become increasingly important with increasing stream size. We predicted that water column nutrient uptake increases with stream size, mirroring carbon/energy dynamics predicted by the RCC. We measured water column uptake of ammonium ( $${\text{NH}}_{4}^{ + }$$ ), nitrate ( $${\text{NO}}_{3}^{ - }$$ ), and soluble reactive phosphorus (SRP) in 1st through 5th order stream and river reaches (discharge: 50–68,000 L s−1) in three watersheds ranging from 70 % developed lands. We found that water column volumetric uptake (U vol ) of $${\text{NH}}_{4}^{ + }$$ , $${\text{NO}}_{3}^{ - }$$ , and SRP did not significantly differ among watersheds and we did not find any longitudinal patterns for U vol . Uptake velocity (v f ) of $${\text{NH}}_{4}^{ + }$$ increased with stream size, whereas $${\text{NO}}_{3}^{ - }$$ and SRP v f did not differ with stream size or among watersheds. Both U vol and v f were related to water column metabolism and material suspended in the water column, but specific relationships differed among solutes and uptake metrics. Median water column v f across 15 sites was 4, 9, and 19 % of median whole-stream $${\text{NH}}_{4}^{ + }$$ , $${\text{NO}}_{3}^{ - }$$ , and SRP v f based upon a previous meta-analysis. Thus, although we could not demonstrate a generalized longitudinal pattern in water column nutrient uptake, water column processes can be important. An improved mechanistic understanding of the controls on uptake and the ultimate fate of nutrients will facilitate effective management and restoration for mitigating downstream nutrient export.

Hydrologic, metabolic and chemical regulation of water-column metabolism and atmospheric CO2 exchange in a large continental reservoir during spring and summer

Abstract Large continental reservoirs are important sinks of terrestrially-derived carbon (C), yet exhibit high inter-annual variability in net CO 2 exchange with the atmosphere due to incompletely understood interactions between supply of solutes (C, nutrients, pH) and lake metabolism. Here we quantified the interaction between hydrology, aquatic metabolism and atmospheric CO 2 exchange in the lacustrine Qu'Appelle arm of Lake Diefenbaker, Canada, by comparing total phytoplankton abundance (as Chlorophyll a ), net primary productivity (NPP), ratios of microbial productivity (P) to respiration (R), and chemically-enhanced CO 2 flux over four springs and summers (2010–2013) with contrasting hydrologic characteristics. Overall, dissolved oxygen (DO) concentrations were correlated inversely ( R 2 adj = 0.63, p 18 O values of DO indicating a strong metabolic coupling in this polymictic basin, whereas correlations between concentration and C isotope ratios of dissolved inorganic carbon (DIC) were not significant ( R 2 adj 1, the basin was slightly autotrophic and CO 2 was captured from the atmosphere. In contrast, the Qu'Appelle arm was a substantial source of CO 2 during 2012 when river discharge was low, pH was 2 exchange ( R 2 adj CO2-pH = 0.84, p 2 dynamics on seasonal and inter-annual scales.

Plankton community respiration, net ecosystem metabolism, and oxygen dynamics on the Louisiana continental shelf: Implications for hypoxia

We conducted a multi-year study of the Louisiana continental shelf (LCS) to better understand the linkages between water column metabolism and the formation of hypoxia (dissolved oxygen <2mlO2l−1) in the region. Water column community respiration rates (WR) were measured on 10 cruises during spring, summer and fall seasons from 2003 to 2007 at multiple sites distributed across the Louisiana continental shelf, overlapping the region where bottom-water hypoxia occurs. We found consistent broad scale patterns in WR rates that followed depth and salinity gradients across the shelf. Observed WR rates were highest at low salinity inner shelf stations (<30m depth) and decreased with increasing water depth. Surface waters had higher WR rates than bottom waters, a pattern most pronounced near the Mississippi river during spring and early summer. Surface water WR rates were highest in eastern transects and decreased westward; a trend that was not evident in bottom waters. WR tended to be higher in spring and summer compared to fall months, but overall the seasonal variability was small. We combined the WR rate measurements with contemporaneous measurements of phytoplankton productivity rates (reported in Lehrter et al., 2009, Continental Shelf Research, 29: 1861–1872) to estimate net water column metabolism. There was consistent evidence of net heterotrophy, particularly in western transects, and in deeper waters (>40m depth), indicating a net organic carbon deficit on the LCS. We offer a simple scale argument to suggest that riverine and inshore coastal waters may be significant sources of organic carbon to account for this deficit. This study provided unprecedented, continental shelf scale coverage of heterotrophic metabolism, which is useful for constraining models of oxygen, carbon, and nutrient dynamics along the LCS.

Physicochemical and biological factors controlling water column metabolism in Sundarbans estuary, India

BackgroundSundarbans is the single largest deltaic mangrove forest in the world, formed at estuarine phase of the Ganges - Brahmaputra river system. Primary productivity of marine and coastal phytoplankton contributes to 15% of global oceanic production. But unfortunately estuarine dynamics of tropical and subtropical estuaries have not yet received proper attention in spite of the fact that they experience considerable anthropogenic interventions and a baseline data is required for any future comparison. This study is an endeavor to this end to estimate the primary productivity (gross and net), community respiration and nitrification rates in different rivers and tidal creeks around Jharkhali island, a part of Sundarbans estuary surrounded by the mangrove forest during a period of three years starting from November’08 to October’11.ResultsVarious physical and chemical parameters of water column like pH, temperature, conductivity, dissolved oxygen, turbidity, suspended particulate matter, secchi disc index, tidal fluctuation and tidal current velocity, standing crop and nutrients were measured along with water column productivity. Relationship of net water column productivity with algal biomass (standing crop), nutrient loading and turbidity were determined experimentally. Correlations of bacterial abundance with community respiration and nitrification rates were also explored. Annual integrated phytoplankton production rate of this tidal estuary was estimated to be 151.07 gC m-2 y-1. Gross primary productivity showed marked inter annual variation being lowest in monsoon and highest in postmonsoon period.ConclusionAverage primary production was a function of nutrient loading and light penetration in the water column. High aquatic turbidity, conductivity and suspended particulate matter were the limiting factors to attenuate light penetration with negative influence on primary production. Community respiration and nitrification rates of the estuary were influenced by the bacterial abundance. The estuary was phosphorus limited in postmonsoon whereas nitrogen-limited in premonsoon and monsoon period. High algal biomass and primary productivity indicated the estuary to be in eutrophic state in most of the time throughout the year. Our study also indicated a seasonal shifting between autotrophic and heterotrophic conditions in Sundarban estuarine ecosystem and it is a tropical, well mixed (high tidal influx) and marine dominated (no fresh water connection) system.

Influences of Multi-Scale Habitat on Metabolism in a Coastal Great Lakes Watershed

Spatial heterogeneity influences ecosystem structure and function across multiple habitat scales. Although primary production and respiration are fundamental to energy cycling in aquatic ecosystems, we know relatively little about how habitat scale influences metabolism. In this study, we adopted a multi-scale habitat approach to evaluate primary production and respiration in a coastal Great Lakes watershed that is experiencing pressure from past, present, and anticipated future human activities. We divided the watershed into five macrohabitats (stream, river, wetland, drowned-river mouth lake, and Great Lake), two mesohabitats (benthic and water column), and four microhabitats (periphytic substrates: sand/sediment, rock, wood, and plant) for evaluation of spatial patterns and synchrony in metabolism. Factors that influenced patterns of metabolism were scale dependent. Algal biomass strongly influenced spatial patterns in metabolism at the meso- and microhabitat scales; greater algal biomass translated to higher areal-specific and lower chlorophyll-specific metabolism at benthic mesohabitat and sand/sediment and rock microhabitats. Benthic metabolism overwhelmed water column metabolism, irrespective of location or time of year. Watershed position was important at the macrohabitat scale, with greater overall metabolism in macrohabitats located lower in the watershed. Average synchrony in metabolism rates was greatest at the macrohabitat scale, suggesting metabolic patterns that are evident at finer scales may become integrated at coarser scales. Our results (1) show that spatial and temporal patterns in metabolism are shaped by factors that are dependent upon habitat scale; (2) highlight the importance of benthic productivity across habitat and season; and (3) suggest that hydrologic connectivity strongly influences ecosystem processes, although physical factors can affect these responses as evidenced by the low levels of synchrony between Lake Michigan and the other macrohabitats.

Using river-scale experiments to inform variable releases from large dams: a case study of emergent adaptive management

Case studies of successful adaptive management generally focus on examples that have frameworks for adaptive management embedded from project conception. In contrast, this paper outlines an example of emergent adaptive management. We describe an approach whereby targeted research and collaboration among stakeholders assisted learning, and ultimately the development of interim operational guidelines for increased within-channel flow variability in the highly regulated Mitta Mitta River, which is managed as part of the River Murray System in the Murray–Darling Basin, Australia. Environmental monitoring of four variable flow trials evaluated the response of water column microbial activity, benthic and water column metabolism, the structure and composition of algal biofilms, and benthic macroinvertebrates to increased flow variability created by varying the release from Dartmouth Reservoir. Each trial built upon lessons from previous trials, with collaboration among key stakeholders occurring before, during and after each trial. Institutional conditions encouraged a shift to adaptive management over time that helped to achieve environmental, social and economic objectives downstream of the dam. A key lesson is that adaptive management does not have to be specified a priori, but can emerge within a trusting relationship between stakeholders as long as they are willing and able to change their operational paradigm.

Origins and scales of hypoxia on the Louisiana shelf: Importance of seasonal plankton dynamics and river nutrients and discharge

Management plans for the Mississippi River Basin call for reductions in nutrient concentrations up to 40% or more to reduce hypoxia in the Gulf of Mexico (GOM), while at the same time the government is considering new farm subsidies to promote development of biofuels from corn. Thus there are possibilities of both increasing and decreasing river nutrients depending on national priorities. River flow rates which also influence the extent of hypoxia on the shelf may be altered by global climate change. We have therefore developed a series of simulations to forecast ecosystem response to alterations in nutrient loading and river flow. We simulate ecosystem response and hypoxia events using a linked model consisting of multiple phytoplankton groups competing for nitrogen, phosphorus and light, zooplankton grazing that is influenced by prey edibility and stoichiometry, sub-pycnocline water-column metabolism that is influenced by sinking fecal pellets and algal cells, and multi-element sediment diagenesis. This model formulation depicts four areas of increasing salinity moving westward away from the Mississippi River point of discharge, where the surface mixed layer, four bottom layers and underlying sediments are represented in each area. The model supports the contention that a 40% decrease in river nutrient will substantially reduce the duration and areal extent of hypoxia on the shelf. But it also suggests that in low and middle salinity areas the hypoxia response is saturated with respect to nutrients, and that in high salinity regions small increases in nutrient and river flow will have disproportionally large effects on GOM hypoxia. The model simulations also suggest that river discharge is a stronger factor influencing hypoxia than river nutrients in the Mississippi River plume. Finally, the model simulations suggest that primary production in the low salinity regions is light limited while primary production in the higher salinity zones is phosphate limited during the May to October period when hypoxia is prevalent in the Mississippi River plume.

Common catabolic enzyme patterns in a microplankton community of the Humboldt Current System off northern and central-south Chile: Malate dehydrogenase activity as an index of water-column metabolism in an oxygen minimum zone

Abstract An extensive subsurface oxygen minimum zone off northern and central-south Chile, associated with the Peru–Chile undercurrent, has important effects on the metabolism of the organisms inhabiting therein. Planktonic species deal with the hypoxic and anoxic environments by relying on biochemical as well as physiological processes related to their anaerobic metabolisms. Here we characterize, for the first time, the potential enzymatic activities involved in the aerobic and anaerobic energy production pathways of microplanktonic organisms (

Origins and temporal scales of hypoxia on the Louisiana shelf: Importance of benthic and sub-pycnocline water metabolism

Hypoxic-to-anoxic conditions (2–0 mg O 2 l − 1 ) occur in the bottom waters of the northern Gulf of Mexico on the Louisiana shelf west of the Mississippi river delta during late spring and summer where the rate of oxygen consumption exceeds its rate of input from physical transport plus photosynthetic generation. Although consumption of oxygen in the water column primarily via oxic respiration is an important process, the loss of oxygen at and near the seafloor may also be an important sink contributing to seasonal low oxygen conditions in the relatively shallow overlying waters in this region. Associated with the flux of oxygen into the sediments is the flux of nutrients out of the sediments from the remineralization of sedimentary organic matter via a number of possible electron acceptors. The nutrients that are released from the sediment can potentially stimulate further primary production. This can lead to generation of oxygen in the water column and production of organic matter, much of which can be transported to the seafloor where it again becomes a sink for oxygen. A non-steady-state data driven numeric benthic–pelagic model was developed to investigate the role of sediment and water-column metabolism in the development of hypoxia on the Louisiana shelf. The model simulations bare out the importance of sediment oxygen demand as the primary sink for oxygen at the beginning and end of a hypoxic event on the shelf, but once hypoxia has developed, the sediments, now isolated from the oxygen-rich surface waters, are driven into a more anoxic mode, becoming more dependent on sulfate and metal reduction. As a result, the bottom water near the pycnocline becomes the major sink for oxygen. Model simulations also suggest that there is a delay of several weeks between metabolite production (especially ammonium) and its efflux from the sediments. Thus the maximum sediment ammonium export occurs in September and October in time to fuel autumnal phytoplankton production, thereby continuing a biogeochemical cycle that expands the temporal and spatial scales of hypoxia on the Louisiana shelf.

Aquatic metabolism as an indicator of the ecological effects of hydrologic pulsing in flow-through wetlands

Abstract Water column metabolism is a major component in the functioning of wetland ecosystems and can be used as an indicator of ecosystem health. The effect of hydrologic pulsing on water column metabolism was studied with 2 year's field data and a validated model for two 1-ha created riparian wetlands in Columbus, OH, USA. Aquatic gross primary productivity (GPP) was measured during hydrologic pulses the first week of April, May and June of 2004 and compared to GPP during steady flow-conditions in April, May and June 2005. Pulses reduced diurnal variation of water temperature, pH and dissolved oxygen, and negatively affected GPP rates. Mean GPP measured during hydrologic flood pulses was 5.4 ± 2.6 kcal m −2 day −1 , significantly lower than that measured for comparable months with steady-flow hydrology (10.8 ± 3.3 kcal m −2 day −1 ). Solar-normalized productivity values of 0.08 ± 0.01% of solar energy during pulses and 0.2 ± 0.02% for steady-flow conditions were also significantly different. Different hyperbolic curves of optimum productivity with water temperature were seen for pulsing and steady-flow conditions. A simulation model with hydrology, metabolism, and dissolved oxygen sub models was calibrated with 2005 steady-flow year data and validated with 2004 pulse year data. Results from both the field study and model simulations suggest that there was a threshold hydraulic inflow rate between 30 and 50 cm day −1 where aquatic metabolism became negatively affected by flow.

Magnitude and variability of benthic and pelagic metabolism in a temperate coastal lagoon

In shallow coastal systems where most of the seafloor lies within the photic zone, ben- thic photoautotrophy is likely to play a key role in regulating carbon and nitrogen cycling. We mea- sured dissolved inorganic carbon exchanges in seasonal microcosm incubations to determine the rel- ative importance of benthic (with and without macroalgae) and water column metabolism at 3 sites located along a nutrient gradient in a coastal barrier-island lagoon on the eastern shore of Virginia. When coupled with data on in situ biomass, the incubations clearly indicated a seasonality in the dominance of primary producers at the sites, with benthic primary producers being the most impor- tant early and late in the growing season and phytoplankton dominating following a mid-summer period of macroalgal decay and the release of algal-bound nutrients to the water column. The ben- thos was always net autotrophic, and differences in community metabolism at the 3 sites appeared to be largely due to variation in macroalgal biomass. Macroalgae accumulated to a greater extent in the mid-lagoon and accounted for up to 96% of benthic production. The water column was net hetero- trophic throughout the lagoon except following the mid-summer macroalgal collapse, when phyto- plankton biomass was highest at the 2 sites closest to the mainland. Benthic microalgal production also increased in importance following the macroalgal decline, suggesting that competition for light and possibly nutrients limited benthic microalgal production when macroalgal densities were high. Overall, there was a distinct seasonality in patterns of total metabolism within the bay, where all sites were net autotrophic in the spring and summer and net heterotrophic in the fall. Up to 8 g N m -2 accu- mulated in macroalgal biomass throughout spring and early summer at the mid-lagoon shoal site, slowing nutrient transport through the lagoon. Nutrient turnover rates were higher during the period when phytoplankton and benthic microalgae were the dominant primary producers. This study illus- trates the functional importance of different primary producer communities to carbon metabolism and to the temporary retention of nutrients in lagoonal land-margin ecosystems.

Phytoplankton Production and Seasonal Biomass Variation of Seagrass, Ruppia maritima L., in a Tropical Mexican Lagoon with an Ephemeral Inlet

Plankton metabolism andRuppia maritima biomass were measured seasonally during 1982–83 in El Verde Lagoon, a small coastal lagoon with an ephemeral inlet on the Pacific Coast of Mexico. Total net aquatic primary production was 521 g C m−2 y−1. The water column was slightly heterotrophic, with an annual P/R ratio of 0.89. Our analysis indicates that tropical and subtropical coastal lagoons with restricted or seasonal inlets have generally higher net aquatic primary productivity levels than lagoons with permanently open inlets. We hypothesize that this is due to retention of nutrients and plankton stocks during the dry season. The seasonal pattern of water column metabolism was related to rainfall and riverflow, with higher values generally occurring during the wet season. Net production and respiration were about three times lower during the 1982 dry season as compared to the 1983 dry season which received considerable rains due to abnormal climatic conditions. The biomass ofR. maritima ranged from zero to 620 g dry wt m−2. Growth occurred only during the dry season and there were two distinct biomass peaks representing two separate crops. The second crop was heavily epiphytized with nitrogen-fixing algae. There was an apparent succession in dominance of water column productivity over the year, withRuppia dominating during the dry season and phytoplankton more important during the wet season.