Benthic Gross Primary Production Research Articles

Depth and basin shape constrain ecosystem metabolism in lakes dominated by benthic primary producers

AbstractMetabolism is one of the most fundamental ecosystem processes, but the drivers of variation in metabolic rates among lakes dominated by benthic primary producers remain poorly constrained. Here, we report the magnitudes and potential drivers of whole‐lake metabolism across 43 Swedish arctic–alpine lakes, based on the free‐water diel oxygen technique with sondes deployed during the open‐water season near the surface and bottom of the lakes. Gross primary production (GPP) and ecosystem respiration (R) were strongly coupled and ranged from 0.06 to 0.45 mg and 0.05 to 0.43 mg L−1 d−1 among lakes. On average, GPP and R decreased eightfold from relatively shallow to deep lakes (mean depth 0.5–10.9 m) and twofold from concave to convex lakes (mean depth: maximum depth 0.2–0.5). We attribute this to light limitation and shape‐specific sensitivity of benthic GPP to disturbance by lake ice. Net ecosystem production (GPP‐R) ranged from −0.09 to 0.14 mg L−1 d−1 and switched, on average, from positive to negative towards deeper lakes and lakes richer in dissolved organic carbon (DOC; 0.5–7.4 mg DOC L−1). Uncertainties in metabolism estimates were high (around one and three times mean R and GPP), especially in deep lakes with low insulation and diurnally variable wind speed. Our results confirm the role of DOC in stimulating net heterotrophy and highlight novel effects of lake shape on productivity in benthic‐dominated lake ecosystems and its response to changes in lake ice cover.

An ecological and methodological assessment of benthic gross primary production in northern lakes

AbstractBenthic gross primary production (GPP) is often the most important part of aquatic food webs in northern lakes, which are gradually warming and receiving increased terrestrial colored dissolved organic carbon loadings due to global change. Yet, measurements of benthic GPP are fairly uncommon, and methods and unit dimensions of benthic GPP are unstandardized and rarely compared. In this study, we measured benthic GPP in 27 headwater lakes from three regions in northern Sweden and analyzed potential constraining drivers of benthic GPPz rates at discrete depths and estimates of benthic GPP averages across the whole lake, as well as across the littoral zone. We also compared in situ measurements of benthic GPP averages across the whole lake with modeled values using the “autotrophic structuring model.” We found that benthic GPPz rates were best explained by, and positively related to, available light (i.e., a function of depth and water color) and temperature. Benthic GPP averages across the whole lake, on the contrary, were best explained by the relative size of the littoral area, which is a measure that combines lake bathymetry and water color. The comparison between in situ measured and modeled estimates of benthic GPP averages across the whole lake revealed that (1) the autotrophic structuring model underestimates GPP at low values and overestimates GPP at high values compared with measured data, and that (2) measured values were related to temperature, which is not included as a variable in the autotrophic structuring model. Considering future predicted changes impacting northern latitude lakes, our results suggest that increased lake water temperatures can to some extent mitigate the negative impacts of reduced light availability from lake browning on benthic GPPz rates. The combined impact of these changes on benthic GPP averages across the whole lake will depend on, and be moderated by, lake bathymetry determining the relative size of the littoral area.

Contribution of boulder reef habitats to oxygen dynamics of a shallow estuary

We assessed the importance of boulder reefs to the oxygen dynamics of a shallow estuary during two growing seasons in 2017 and 2018. Using open-system diel oxygen measurements and benthic and pelagic incubations, we evaluated the relative contribution of pelagic and benthic habitats to the ecosystem metabolism along a depth gradient in two areas, with (Reef) and without (Bare) boulder reefs in the Limfjorden, Denmark. System integrated areal rates of gross primary production (GPP) and ecosystem respiration (ER) both increased with depth in both areas. Benthic contribution to system GPP and ER was highest at shallow depth where it represented 47 and 53% respectively. However, with increasing depth pelagic processes dominated GPP and ER (98 and 94%) even in the Reef area. Although the Reef area had higher biomass of auto- and heterotrophic organisms (macroalgae and macrofauna), benthic GPP was at similar level in both areas, due to a significant contribution from micro-phytobenthic organisms. The Reef area had lower sediment pools of organic matter, nitrogen and phosphorous and was slightly more oxygenated compared to the nearby Bare area. Extreme temperatures and higher levels of nutrients in 2018 caused a marked increase in benthic ER rates resulting in net heterotrophy (NEM = GPP − ER < 0) in 2018 compared to net autotrophy (NEM > 0) in 2017. Under current eutrophic conditions, boulder reefs do not contribute positively to the oxygen dynamics in the estuary. Reoccurring blooms of phytoplankton with high organic matter decomposition combined with high temperatures and dominance of fauna stimulate depletion of oxygen around the reefs. Significant improvements in water clarity are needed to regrow perennial macroalgae and induce net autotrophy. Under current turbid conditions, it is only recommended to establish boulder reefs in shallow (<3 m) parts of the estuary.

Incomplete recovery of a shallow lake from a natural browning event

Abstract Terrestrial loads of dissolved organic carbon (DOC) have increased in recent years in many north temperate lakes, leading to widespread brownification. This process can profoundly alter the relative strength of planktonic and benthic primary production in lakes due to changes in light and nutrient availability. While most existing in situ studies of brownification focus on gradual decadal trends, short‐term flooding can induce rather sudden natural browning events in lakes. We investigated the recovery of a small, temperate, groundwater‐fed shallow lake from a strong natural flooding‐induced browning and nutrient loading event. We hypothesised that along with a return to pre‐flood lake water levels, DOC concentrations would return to pre‐flood concentrations due to bacterial and photolytic mineralisation and flocculation, while total phosphorus (TP) concentrations would show a delayed and potentially incomplete recovery due to sustaining effects of phytoplankton dominance on nutrient dynamics in the lake. Phytoplankton gross primary production (GPP), which surged in response to higher nutrient concentrations and a compressed mixing depth, was expected to follow the same recovery pattern as TP. Benthic periphyton GPP was expected to recover in tandem with improving light conditions. We measured post‐flood water levels, water chemistry, and summer pelagic and benthic GPP. The lake water level declined gradually, returning to pre‐flood levels within 4 years. As hypothesised, DOC concentrations initially declined rapidly, but remained 1.5‐fold higher (c. 22 mg/L) than pre‐flood values 3 years after peak browning. TP concentrations fluctuated greatly between seasons but remained generally elevated in subsequent years, being roughly double pre‐browning values (c. 130 µg/L) 3 years post‐flood. Measurements taken in 2019, 7 years after the initial browning event, showed both DOC and TP concentrations remaining at these elevated levels. Across the intensive study period 3 years before and after peak browning (2010–2015), TP concentrations were positively correlated to summer phytoplankton biomass, while DOC and TP concentrations were negatively correlated to summer periphyton biomass and GPP. Accordingly, the lake's recovery period exhibited a decline of pelagic GPP and a partial recovery of summer periphyton biomass and production, although a full return to pre‐flood values was not recorded in either case. Whole‐lake areal summertime GPP increased due to browning and remained higher than before the browning event. This study provides a rare examination of differences in response rates between DOC, TP, and autotrophic structure in a shallow, temperate lake following a natural flood‐induced extreme browning event. The available data provide compelling evidence that browning events can produce lasting changes in the water chemistry, trophic status, and productive pathways of a lake, persisting beyond the lake's return to pre‐flood water levels. Such delayed or incomplete recoveries from flooding‐induced browning and nutrient loading may have consequences for regional and long‐term temporal trends in lake biogeochemistry, potentially contributing to the widespread phenomenon of incremental brownification.

An experimental test of climate change effects in northern lakes: Increasing allochthonous organic matter and warming alters autumn primary production

Abstract Climate changes are predicted to influence gross primary production (GPP) of lakes directly through warming and indirectly through increased loads of allochthonous coloured dissolved organic matter (cDOM) from surrounding landscapes. However, few studies have investigated this combined effect. Here we tested the effects of warming (elevated 3℃) and cDOM input (three levels of humic river water addition) on GPP in autumn (2 months including open water and ice‐covered periods) in experimental pond ecosystems. The cDOM input decreased whole‐ecosystem GPP at natural temperature conditions mainly as a result of lower benthic GPP not fully counteracted by an increase in pelagic GPP, while warming increased whole‐ecosystem GPP due to a positive response of mainly pelagic GPP at all levels of cDOM input. Warming delayed autumn ice cover formation by 2 weeks but did not affect light availability in the water column compared to ambient ice‐covered treatments. Gross primary production during this period was still affected by warming and cDOM. The results stress the importance of accounting for multiple climate drivers and habitats when predicting lake GPP responses to climate change. We conclude that climate change may shift whole‐ecosystem GPP through different responses of habitat‐specific GPP to increasing cDOM inputs and warming.

Shifts in the partitioning of benthic and pelagic primary production within and across summers in Lake Mývatn, Iceland

ABSTRACT The relative contributions of benthic and pelagic primary production affect ecosystem function, but studies documenting natural variation in the partitioning of production (i.e., autotrophic structure) are uncommon. This study examines autotrophic structure of shallow Lake Mývatn over 7 summers (2012–2018). We used routine measurements of benthic gross primary production (GPP), pelagic chlorophyll a concentrations, and pelagic production to estimate benthic and pelagic maximum productivity (P max) across summers. With these parameters and corresponding incident light and water clarity data, we estimated in situ benthic and pelagic GPP. Our results demonstrated substantial variation in Mývatn’s autotrophic structure within and across summers. Benthic GPP often exceeded pelagic GPP. However, periods of increased phytoplankton abundance were associated with increased light attenuation and, consequently, declines in benthic GPP. These effects were strongest in 3 summers with dense cyanobacteria blooms, in which the benthic fraction of total production declined from >95% to <20%. Reduced light levels over a 2-week period were associated with low benthic P max, implying that past shading by phytoplankton may decrease the photosynthetic potential of benthic producers. Moreover, variation in estimated benthic P max values can affect the point at which autotrophic structure shifts toward pelagic-dominated conditions. Overall, our study demonstrates that the balance between benthic and pelagic production can vary greatly at intra- and interannual scales because of changes in the photosynthetic capacity of both pelagic and benthic primary producers. Understanding natural variation in lake autotrophic structure may inform how benthic and pelagic production respond to ongoing and future environmental changes.

Pelagic contribution to gross primary production dynamics in shallow areas of York River, VA, U.S.A.

AbstractThe pelagic contribution to total gross primary production (GPP) is often used to evaluate the assemblages of primary producers in shallow estuarine and coastal ecosystems that shift between benthic‐dominated and pelagic‐dominated status as a response to nutrient loading change. This study investigated the spatial and temporal variability of the pelagic contribution to ecosystem GPP for the years 2003–2015 in shallow areas of the York River. The monthly average values of ecosystem GPP and pelagic GPP were estimated by using 15‐min observational data of dissolved oxygen and chlorophyll a, respectively. A larger pelagic contribution, generally, corresponded to a higher pelagic GPP, while pelagic GPP showed significant positive correlations to riverine discharge and total nitrogen loading at mesohaline and polyhaline stations. The interannual variability in benthic GPP (ecosystem GPP – pelagic GPP), however, exhibited a non‐monotonic relationship to the increase in pelagic GPP. A conceptual model with four ecosystem stages in response to nutrient loading changes is proposed based on changes in the relationship between benthic GPP and pelagic GPP, and that between pelagic contribution and pelagic GPP. The model can explain the different responses of an ecosystem to nutrient loading observed in various estuaries and can be used to evaluate the status of an ecosystem undergoing the eutrophication/oligotrophication process. The stages of York River ecosystems during the study period were quantitatively examined. Results suggest that benthic GPP shows negative correlations to pelagic GPP in the downstream York River over most months of the year, indicating a gradual degradation of the benthic community.

Positive feedback between chironomids and algae creates net mutualism between benthic primary consumers and producers.

The chironomids of Lake Mývatn show extreme population fluctuations that affect most aspects of the lake ecosystem. During periods of high chironomid densities, chironomid larvae comprise over 90% of aquatic secondary production. Here, we show that chironomid larvae substantially stimulate benthic gross primary production (GPP) and net primary production (NPP), despite consuming benthic algae. Benthic GPP in experimental mesocosms with 140,000 larvae/m2 was 71% higher than in mesocosms with no larvae. Similarly, chlorophyll a concentrations in mesocosms increased significantly over the range of larval densities. Furthermore, larvae showed increased growth rates at higher densities, possibly due to greater benthic algal availability in these treatments. We investigated the hypothesis that larvae promote benthic algal growth by alleviating nutrient limitation, and found that (1)larvae have the potential to cycle the entire yearly external loadings of nitrogen and phosphorus during the growing season, and (2) chlorophyll a concentrations were significantly greater in close proximity to larvae (on larval tubes). The positive feedback between chironomid larvae and benthic algae generated a net mutualism between the primary consumer and primary producer trophic levels in the benthic ecosystem. Thus, our results give an example in which unexpected positive feedbacks can lead to both high primary and high secondary production.

Budget of Primary Production and Dinitrogen Fixation in a Highly Seasonal Red Sea Coral Reef

Biological dinitrogen (N2) fixation (diazotrophy, BNF) relieves marine primary producers of nitrogen (N) limitation in a large part of the world oceans. N concentrations are particularly low in tropical regions where coral reefs are located, and N is therefore a key limiting nutrient for these productive ecosystems. In this context, the importance of diazotrophy for reef productivity is still not resolved, with studies up to now lacking organismal and seasonal resolution. Here, we present a budget of gross primary production (GPP) and BNF for a highly seasonal Red Sea fringing reef, based on ecophysiological and benthic cover measurements combined with geospatial analyses. Benthic GPP varied from 215 to 262 mmol C m−2 reef d−1, with hard corals making the largest contribution (41–76%). Diazotrophy was omnipresent in space and time, and benthic BNF varied from 0.16 to 0.92 mmol N m−2 reef d−1. Planktonic GPP and BNF rates were respectively approximately 60- and 20-fold lower than those of the benthos, emphasizing the importance of the benthic compartment in reef biogeochemical cycling. BNF showed higher sensitivity to seasonality than GPP, implying greater climatic control on reef BNF. Up to about 20% of net reef primary production could be supported by BNF during summer, suggesting a strong biogeochemical coupling between diazotrophy and the reef carbon cycle.

Turbidity, light, temperature, and hydropeaking control primary productivity in the Colorado River, Grand Canyon

Dams and river regulation greatly alter the downstream environment for gross primary production (GPP) because of changes in water clarity, flow, and temperature regimes. We estimated reach-scale GPP in five locations of the regulated Colorado River in Grand Canyon using an open channel model of dissolved oxygen. Benthic GPP dominates in Grand Canyon due to fast transport times and low pelagic algal biomass. In one location, we used a 738 days time series of GPP to identify the relative contribution of different physical controls of GPP. We developed both linear and semimechanistic time series models that account for unmeasured temporal covariance due to factors such as algal biomass dynamics. GPP varied from 0 g O2 m−2 d−1 to 3.0 g O2 m−2 d−1 with a relatively low annual average of 0.8 g O2 m−2 d−1. Semimechanistic models fit the data better than linear models and demonstrated that variation in turbidity primarily controlled GPP. Lower solar insolation during winter and from cloud cover lowered GPP much further. Hydropeaking lowered GPP but only during turbid conditions. Using the best model and parameter values, the model accurately predicted seasonal estimates of GPP at 3 of 4 upriver sites and outperformed the linear model at all sites; discrepancies were likely from higher algal biomass at upstream sites. This modeling approach can predict how changes in physical controls will affect relative rates of GPP throughout the 385 km segment of the Colorado River in Grand Canyon and can be easily applied to other streams and rivers.

Seasonal rates of benthic primary production in a Greenland fjord measured by aquatic eddy correlation

We present the first year‐round estimates of benthic primary production at four contrasting shallow (3–22 m depth) benthic habitats in a southwest Greenland fjord. In situ measurements were performed using the noninvasive aquatic eddy‐correlation (EC) oxygen (O2) flux method. A series of high‐quality multiple‐day EC data sets document the presence of a year‐round productive benthic phototrophic community. The shallow‐water sites were on average autotrophic during the spring and summer months, up to 43.6 mmol O2 m−2 d−1, and heterotrophic or close to metabolic balance during the autumn and winter. Substantial benthic gross primary production (GPP) was measured year‐round. The highest GPP rates were measured during the spring, up to 5.7 mmol O2 m−2 h−1 (136.8 mmol O2 m−2 d−1), and even at low light levels (&lt; 80 µmol quanta m−2 s−1) during late autumn and winter we measured rates of up to 1.8 mmol O2 m−2 h−1 (43.2 mmol O2 m−2 d−1) during peak irradiance. The benthic phototrophic communities responded seasonally to ambient light levels and exhibited year‐round high photosynthetic efficiency. In situ downwelling irradiances as low as ∼ 2 µmol quanta m−2 s−1 induced an autotrophic response and light saturation indices (Ik) were as low as 11 µmol quanta m−2 s−1 in the winter. On an annual timescale, the average areal rate of benthic GPP was 11.5 mol O2 m−2 yr−1, which is ∼ 1.4 times higher than the integrated gross pelagic primary production of the ∼ 30–50 m deep photic zone of the fjord. These results document the importance of benthic photosynthesis on an ecosystem level and indicate that the benthic phototrophic compartment should be accounted for when assessing carbon and nutrient budgets as well as responses of coastal Arctic ecosystems to climate change.

Benthic and planktonic primary production along a nutrient gradient in Green Bay, Lake Michigan, USA

AbstractPrimary production in lakes occurs in both planktonic (water column) and benthic (bottom) habitats. How whole-lake primary production is distributed between these 2 habitats—referred to as autotrophic structure—is a key ecosystem property. Empirical research examining the balance between benthic and planktonic primary production in lakes is scarce, and how autotrophic structure changes across depth, nutrient, water clarity, and biological invasion gradients is unclear. Therefore, we are ill equipped to anticipate ecosystem-level responses to environmental change. We assessed the magnitude of offshore planktonic, nearshore planktonic, and nearshore benthic gross primary production (GPP) along a gradient of nutrients, water clarity, and Cladophora biomass in Green Bay, Lake Michigan, USA, during summer 2010, 2011, and 2012. Benthic and planktonic GPP varied strongly along the trophic gradient. Planktonic GPP increased with nutrients status, whereas benthic GPP decreased. From shore to 10 m depth, au...

Benthic and Pelagic Primary Production in Different Nutrient Regimes

Benthic flora can contribute significantly to gross primary production (GPP) of shallow coastal waters where light reaches the sea bottom. We quantified and compared benthic and pelagic GPP along nutrient gradients in time and space in the shallow estuary, Limfjorden, Denmark, based on monitoring data combined with historical information. Limfjorden experienced a shift from a pristine, benthic-dominated clear water regime with high total GPP in the early twentieth century to a eutrophic, plankton-dominated regime still with high total GPP in the 1980s when nutrient loadings peaked. Recent reductions in nutrient loadings reduced pelagic GPP, particularly in spring, but water clarity and benthic GPP did not increase correspondingly, so total GPP declined. The most nutrient-rich basins have remained plankton-dominated, with benthic vegetation constrained to shallow waters. The results support existing evidence that total GPP of shallow coastal areas does not increase systematically with eutrophication. Furthermore, the results suggest that total GPP may decline temporarily during oligotrophication as pelagic GPP declines, while feedback mechanisms delay or prevent restoration to a state with benthic dominance of GPP.

Benthic metabolism in two turbid dryland rivers

Summary1. Australian dryland rivers have among the most variable discharge of any rivers worldwide and are characterized by extended periods of no flow during which aquatic habitat contracts into isolated waterholes. Despite naturally high turbidity, benthic primary production is known to be the main source of carbon to waterhole food webs. The objective of this study was to quantify rates of benthic metabolism and identify factors influencing these rates in two Australian dryland rivers, the Cooper Creek and the Warrego River.2. Both rivers have similar variable hydrology and high levels of turbidity (photic depths &lt; 0.4 m), but fish abundance in Cooper Creek is 10 times than that of the Warrego River. Therefore, an additional aim of the study was to determine if fish abundances reflected underlying differences in benthic primary production.3. Benthic gross primary production (GPP), benthic respiration, nutrient concentrations and light penetration were measured immediately after flow had ceased (‘post‐flow’) and after at least 2 months of zero flow (‘no‐flow’) in 15 waterholes from each river. A subset of four waterholes from each river was sampled on two additional occasions to determine if patterns were consistent over time.4. Cooper Creek generally had higher rates of GPP and a more autotrophic benthic zone than the Warrego River. As a result, the expected positive relationship between fish abundance and GPP was generally observed at a broad catchment scale.4. Light was the major control in benthic GPP in both rivers, as nutrient concentrations were high on all sampling occasions. However, for similar values of photic depth, GPP was greater in Cooper Creek than in the Warrego River. This suggests that more frequent disturbance of the littoral zone may inhibit biofilm development in waterholes of the Warrego River.5. Although flow variability in dryland rivers is extreme compared with other rivers worldwide, cycles of expansion and contraction of aquatic habitat in these two rivers were associated with a shift in the dominance of regional scale (subcatchments contributing to river flow) versus local scale (waterhole morphology) influences on ecosystem functioning, similar to floodplain rivers in tropical and temperate regions.

Benthic metabolism across a gradient of anthropogenic impact in three shallow coastal lagoons in NW Florida

Seagrasses are being lost at alarming rates worldwide, most often due to anthropogenic effects, but few reports have examined how seagrass loss affects the metabolism of coastal ecosys- tems. Here, we address this question by comparing both areal and system-integrated daytime ben- thic metabolic rates across 3 lagoons in the North Central Gulf of Mexico that display varying levels of abundance of the shoalgrass Halodule wrightii (i.e. from 64% of the bottom covered with shoal- grass down to between 4 and 0%), partially due to contrasting anthropogenic pressures. When com- paring the 2 shoalgrass-containing lagoons, shoalgrass patches featured higher areal rates of gross primary production (GPP) and respiration (R), and, to a lesser extent, higher rates of net production (NP), than did bare sediment. These results were robust despite across-lagoon differences in percent- age cover and areal biomass of shoalgrass and benthic microalgae (i.e. the lagoon with less cover also had lower areal shoalgrass biomass in the shoalgrass patches and higher areal microalgal chlorophyll a concentrations in the sediment). We did not find any consistent differences in the metabolic rates of bare sediment across the 3 lagoons, despite the fact that areal microalgal chlorophyll a concentra- tions in bare sediment increased as shoalgrass abundance decreased across the 3 lagoons. System- integrated rates of benthic GPP and R were higher in the lagoon with the highest shoalgrass cover when compared with the lagoons with little or no shoalgrass; but, surprisingly, system-integrated rates of benthic NP did not differ significantly across lagoons. This result suggests that the large decrease in shoalgrass abundance across the lagoons examined does not greatly affect the lagoon's potential capacity for accumulation and/or export of organic carbon. It also underlines the importance of deriving system-integrated estimates to properly understand how decreasing seagrass abundance can alter the daytime metabolism of coastal systems.

Benthic Metabolism as an Indicator of Stream Ecosystem Health

We tested direct and indirect measures of benthic metabolism as indicators of stream ecosystem health across a known agricultural land-use disturbance gradient in southeast Queensland, Australia. Gross primary production (GPP) and respiration (R-24) in benthic chambers in cobble and sediment habitats, algal biomass (as chlorophyll a) from cobbles and sediment cores, algal biomass accrual on artificial substrates and stable carbon isotope ratios of aquatic plants and benthic sediments were measured at 53 stream sites, ranging from undisturbed subtropical rainforest to catchments where improved pasture and intensive cropping are major land-uses. Rates of benthic GPP and R-24 varied by more than two orders of magnitude across the study gradient. Generalised linear regression modelling explained 80% or more of the variation in these two indicators when sediment and cobble substrate dominated sites were considered separately, and both catchment and reach scale descriptors of the disturbance gradient were important in explaining this variation. Model fits were poor for net daily benthic metabolism (NDM) and production to respiration ratio (P/R). Algal biomass accrual on artificial substrate and stable carbon isotope ratios of aquatic plants and benthic sediment were the best of the indirect indicators, with regression model R-2 values of 50% or greater. Model fits were poor for algal biomass on natural substrates for cobble sites and all sites. None of these indirect measures of benthic metabolism was a good surrogate for measured GPP. Direct measures of benthic metabolism, GPP and R-24, and several indirect measures were good indicators of stream ecosystem health and are recommended in assessing process-related responses to riparian and catchment land use change and the success of ecosystem rehabilitation actions.

Ecosystem metabolism in a subtropical, seagrass-dominated lagoon

Measurements of ecosystem gross primary production (GPP) and respiration (R) were made for 2 to 3 consecutive days on 10 occasions from February 1996 through June 1997 in the seagrass-dominated Laguna Madre (Texas, USA). Ecosystem GPP and R were quantified using 3 independent measurements including the open-water technique using dissolved oxygen (DO) and dissolved inorganic carbon (DIC) and the summation of discrete measurements of the water column and benthos. Die1 curves of DIC were often influenced by water movement while the impact of water movement on DO was not apparent. This was attributed to the longer turnover time of D1C relative to DO in the water column. The ecosystem GPP and R estimated using the open-water method with DO ranged from 56 to 366 and 103 to 381 mm01 C m-Z d-l, respectively. The discrete estimates of GPP and R were usually lower and ranged from 81 to 233 and 54 to 218 mm01 C m-' d-l, respectively. The results of these 2 approaches indicated that Laguna Madre is a very productive system where GPP and R are nearly balanced. Rates of GPP and R for this system were similar to those measured in other studies of temperate and tropical seagrass systems. Discrete measurements of GPP and R indicated that the water colun~n was net heterotrophic most of the year (annu~illy weighted average P:R = 0.27). Measurements of benthic GPP and R indicated that it was net autotrophic during the entire year (annual P:R = 1.16), suggesting that the benthos was responsible for fueling heterotrophic activity in the water column.