Benthic Production Rates Research Articles

Dynamics of microphytobenthos photosynthetic activity along a depth transect in the sandy northeastern Gulf of Mexico shelf

The temporal and spatial dynamics of microphytobenthos activity in sandy shelf sediments are poorly understood, which limits assessments of the role of the benthic production in the coastal carbon cycle. The goal of this two-year time series study in the northeastern Gulf of Mexico therefore was to determine how benthic primary production rates in the West Florida Shelf change over the seasons and water depth. The study took place in the Big Bend region of Florida and used three stations along a transect starting at 5 m water depth and ending 29 km offshore, at 18 m water depth. Multisensor YSI 6600 probes installed at each station recorded key environmental parameters at the seafloor. Sediment cores, collected by SCUBA divers from each of the 3 stations at near-monthly intervals over a two-year time period were incubated in the laboratory for production and consumption rate measurements. Incubation of the cores at 200 μE m−2 s−1 allowed comparison of potential productivity between stations and seasons. Community light response curves, produced from sediment incubations at different light intensities, were used to calculate benthic productivity for measured in-situ light levels. The benthic production rates calculated from the core incubations and those calculated using the community light response curves agreed within a factor of three. Average sediment gross primary production rates based on in-situ light intensities ranged from 48 ± 60 mg C m−2 d−1 at 5 m and 81 ± 104 mg C m−2 d−1 at 10 m, to 13 ± 18 mg C m−2 d−1 at 18 m water depth. The corresponding average sediment consumption rates were −62 ± 36, −50 ± 55, and −29 ± 46 mg C m−2 d−1 at 5, 10 and 18 m water depth respectively. On average, production and consumption in the benthic system were found to be balanced, and the nearshore Stations A and B may temporarily turn net autotrophic during periods of high light intensity at the bottom. Our results suggest that the benthic production in this region may contribute up to 50% to the shelf primary production.

Biogeochemistry of dissolved inorganic carbon and nutrients in seagrass (Zostera noltei) sediments at high and low biomass

Nutrient recycling is highly controlled by benthic macrophytes in shallow coastal zones. In seagrass bed, this recycling mainly occurs in sediment porewater, which is considered to be the main source of nutrients for plant growth. This work investigated the effect of a Zostera noltei meadow on the biogeochemistry of inorganic nutrients and carbon in vegetated sediments of the Arcachon Bay. We characterized the vertical distributions of dissolved inorganic C (DIC), NH4+, dissolved inorganic P (DIP) and redox-sensitive particulate inorganic P in bare sediments and in sediments heavily or sparsely colonised by Z. noltei. The obtained concentration profiles were used to calculate benthic production rates or particulate stocks. The porewater nutrient concentrations measured in vegetated sediments were depleted by a factor of 10–100 compared to bare sediments. The concentrations of NH4+, DIP and DIC exhibited a strong vertical zonation reflecting successive layers of intense production or consumption within the root zone often considered and sampled as a homogeneous zone. The deeper root zone showed higher consumption rates of dissolved inorganic nutrients compared to the upper root zone as young roots growing deeper in the sediment played a major role in nutrient plant uptake. A consumption of DIC was consistently recorded in the root zone. We hypothesized that this could result from a transport of CO2 from roots to leaves supporting photosynthetic activity as suggested for freshwater plants. We pointed out to what extent the influence of seagrasses changed between sparse and dense meadows. This work emphasizes the high spatial variability in biogeochemical processes occurring at small scale within the root zone or between dense and sparse seagrass patches. Such a variability should be considered when dealing with the role of seagrass in biogeochemical cycles, especially in the context of global decline of seagrass.

Dissolved organic carbon concentration controls benthic primary production: Results from in situ chambers in north‐temperate lakes

We evaluated several potential drivers of primary production by benthic algae (periphyton) in north‐temperate lakes. We used continuous dissolved oxygen measurements from in situ benthic chambers to quantify primary production by periphyton at multiple depths across 11 lakes encompassing a broad range of dissolved organic carbon (DOC) and total phosphorous (TP) concentrations. Light‐use efficiency (primary production per unit incident light) was inversely related to average light availability (% of surface light) in 7 of the 11 study lakes, indicating that benthic algal assemblages exhibit photoadaptation, likely through physiological or compositional changes. DOC alone explained 86% of the variability in log‐transformed whole‐lake benthic production rates. TP was not an important driver of benthic production via its effects on nutrient and light availability. This result is contrary to studies in other systems, but may be common in relatively pristine north‐temperate lakes. Our simple empirical model may allow for the prediction of whole‐lake benthic primary production from easily obtained measurements of DOC concentration.

Benthic primary production and nitrogen cycling in Spartina alterniflora marshes: effect of restoration after acute dieback

The sudden and massive Spartina alterniflora dieback at the turn of the millennium generated numerous unanswered questions regarding its mechanistic causes and consequences. This study, conducted during 2007–2008, aimed to elucidate mechanisms of recovery and determine whether recovery was accelerated by replanting efforts. The onset of a severe drought during the summer of 2007, however, provided a potential glimpse into the mechanisms driving dieback events. Study sites were established in two of the hardest hit states, Georgia and Louisiana. Each site had a replicated block design consisting of the following four treatments: reference, dieback, dieback with low density replanting (90 cm spacing), and dieback with high density replanting (30 cm spacing). To assess biogeochemical cycling and ecosystem functioning, we quantified rates of nitrogen fixation, potential nitrification, potential denitrification, and benthic production biannually. All measured process rates decreased following the drought year of 2007. Nitrogen fixation was positively correlated with benthic production rates in Louisiana, while denitrification was positively correlated with benthic production rates in Georgia and Louisiana. The lack of decreased benthic production during the 2007 drought could indicate that benthic microphytes cope with better with drought than plants, but may be outcompeted during non-drought years. Replanting efforts significantly increased ecosystem recovery in Louisiana and to a lesser extent in Georgia.

Mass coral spawning: A natural large‐scale nutrient addition experiment

A mass coral spawning event on the Heron Island reef flat in 2005 provided a unique opportunity to examine the response of a coral reef ecosystem to a large episodic nutrient addition. A post‐major spawning phytoplankton bloom resulted in only a small drawdown of dissolved inorganic phosphorus (DIP minimum = 0.37 mmol L−1), compared with almost complete removal of dissolved inorganic nitrogen (DIN) (minimum NO3− = 0.01 mmol L−1; NH4+ = 0.11 mmol L−1), suggesting that pelagic primary production is potentially N limited on the timescale of this study. DIN, DIP, dissolved organic nitrogen (DON), and dissolved organic phosphorus were used in the production of biomass, and mass balance calculations highlighted the importance of organic forms of N and P for benthic and pelagic production in tropical coral reef environments characterized by low inorganic N and P. The input of N and P via the deposition of coral spawn and associated phytodetritus resulted in large changes to N cycling in the sediments, but only small changes to P cycling, because of the buffering capacity provided by the large pool of bioavailable P. It is most likely that this large pool of bioavailable P in the sediments drives potential N limitation of benthic coral reef communities. For example, there was sufficient bioavailable P stored in the top 10 cm of the sediment column to sustain the prespawning rates of benthic production for over 200 d. Most of the change in benthic N cycling occurred via DON and N2 pathways, driven by changes in the quantity and quality of organic matter deposited and decomposed post‐major spawning. The heterotrophic and autotrophic microbial communities within the coral reef sands were able to rapidly (6 to 7 d) process the large episodic load of N and P provided by coral mass spawning.

Benthic nutrient fluxes in euphotic sediments along shallow sub-tropical estuaries, northern New South Wales, Australia

Diurnal benthic fluxes of dissolved inorganic and organic nitrogen (DIN and DON) and dinitrogen gas (N2) were measured in euphotic sediments of 3 shallow sub-tropical Australian estuaries during 4 seasons. The estuaries included 2 impacted by sewage effluent (Brunswick and Simpsons estuaries) and 1 relatively pristine system (Sandon estuary). Sediments acted predomi- nantly as net sinks of DIN throughout the year, except in the nutrient-enriched upper reaches of the Brunswick estuary, where large effluxes of NH4 + occurred during the summer wet season. Distinct light/dark variations in NH4 + fluxes with reduced effluxes or reversal to uptakes occurred during the light in productive sediments. NO3 - was predominantly taken up by sediments at rates proportional to ambient concentrations in the water column. DON commonly comprised a major fraction of fluxes and was controlled primarily by heterotrophic processes in the upper to middle estuaries and autotrophic processes in the lower estuaries. Large deficits in the amount of remineralised DIN (assuming the breakdown of 'Redfield' algae) indicated that a significant amount of nitrogen is either denitrified or immobilised in biomass in the sediments. Benthic fluxes of N2 suggest that denitrifica- tion accounts for a relatively small fraction of the missing nitrogen and immobilisation in biomass (and flow up the food chain) is a potentially major pathway of nitrogen removal in these estuaries. Significant rates of benthic productivity also stimulated secondary heterotrophic production, promot- ing competition for nutrient resources in the sediments. The highest rates of DIN uptake coincided with maximum metabolic rates in sediments where the diurnal p/r (gross productivity/respiration) was between 0.5 and 1, suggesting a peak in competition at this metabolic state. Denitrification rates were lowest at p/r 0.5 to 1 and strongly related to uptake of DIN from the water column, suggesting that competition may cause NO3 - limitation in these euphotic sediments. Dissimilatory NO3 - reduc- tion to ammonium may become relatively more important as water column oxygen saturation drops below 40%, a condition that occurs regularly in the upper Brunswick estuary. NH4 + was only effluxed in net heterotrophic sediments (p/r 1). A conceptual model for benthic nutrient cycling in shallow sub-tropical estuaries is proposed whereby benthic productivity favours to the immobilisation of nitrogen in biomass at the expense of denitrification and recycling back to the water column.

Organic matter and benthic metabolism in euphotic sediments along shallow sub-tropical estuaries, northern New South Wales, Australia

Sediment organic matter (OM), chloropigments and benthic metabolism (oxygen, carbon dioxide and alkalinity fluxes) were investigated along 3 sub-tropical Australian estuaries during different seasons. Significant rates of benthic productivity occurred throughout the year in the study estuaries, especially in the lower reaches where sediments were commonly net autotrophic. Benthic microalgae (BMA) contributed up to 20% of the total carbon content in the lower estuary sands, while organic matter was dominated by phytodetritus and allochthonous material in the middle to upper estuaries. Sediment oxygen demand (SOD) incubations were carried out on surface sediments to investigate labile carbon contents. SOD normalised to sediment carbon content (SOD mol C -1 ) confirmed a much higher proportion of labile OM in the lower estuary sediments due to a predominance of living BMA, while the OM pool in the upper and middle estuaries was relatively more degraded. SOD mol C -1 correlated well with both the C:N and the chlorophyll:pheophytin ratios of surface sediments, providing 3 independent measures of sediment OM quality. Comparisons between SOD and benthic community respiration rates (O 2 fluxes) measured in sediment core incubations suggested that oxygen demand due to the oxidation of reduced sulphur compounds formed a significant fraction of the O 2 flux, and that fluxes may be enhanced by bioturbation. O 2 and TCO 2 respiration rates were overall poorly correlated and suggested that benthic metabolism is characterised by periods of anaerobic mineralisation (TCO 2 :O 2 > 2) during periods of high OM supply, followed by aerobic respiration and reoxidation (TCO 2 :O 2 1 to 2) as OM supply waned. At many sites throughout the study TCO 2 :O 2 flux ratios were less than 1 and coupled with alkalinity uptakes, indicating that either sulphide oxidation or nitrification may be significant influences on the benthic fluxes. These processes cause a net drawdown on O 2 effluxes that can lead to significant underestimations of benthic productivity. As such, the use of CO 2 and O 2 fluxes yield distinctly different measures of benthic metabolism.

THE INFLUENCE OF SUBSTRATE HETEROGENEITY ON BIOFILM METABOLISM IN A STREAM ECOSYSTEM

Simplification of natural habitats is a growing global concern demanding that ecologists better understand how habitat heterogeneity influences the structure and functioning of ecosystems. While there is extensive evidence that physical habitat hetero- geneity affects the structure of biotic communities (i.e., organismal abundance, distribution, diversity, etc.), ecologists know little about how variability in physical conditions within habitats regulates ecological processes that are important for the functioning of an eco- system. We performed a field experiment to assess the effects of geomorphic heterogeneity (i.e., variation in substrate size) on rates of benthic productivity and respiration at the scale of whole riffle habitats in a stream ecosystem. While holding median sizes constant, we manipulated variation in the size of stream bed sediments in replicate riffles to create two treatments representing increased and decreased levels of physical habitat heterogeneity relative to natural conditions in the stream. Physical habitat heterogeneity had an immediate and significant impact on the primary productivity of stream algae and on the respiration of the benthic biofilm. The rates of both ecological processes were elevated in the high- heterogeneity riffles, probably as a result of quantified alterations to near-bed flow velocity and turbulence intensity. Results presented here provide support for the widely held, but largely untested, assumption that physical habitat heterogeneity exhibits control over eco- system-level processes, and it suggests that human-induced simplification of habitats may indeed be altering the functioning of ecosystems.

Microphytobenthos production in the Gulf of Fos, French Mediterranean coast

Microphytobenthic oxygen production was studied in the Gulf of Fos (French Mediterranean coast) during 1991/1992 using transparent and dark benthic chambers. Nine stations were chosen in depths ranging from 0.5 to 13 m, which represents more than 60% of bottoms in the Gulf. Positive net microphytobenthic oxygen production was seasonally detected down to 13 m; the maximum value attained was 60 mg O2 m−2 h−1 (0.7–0.8 g O2 m−2 d−1) in sediments at 0.5 m depth during spring and winter. Respiration rates were maximum in the sediments located at the mussel farm (5 m), in the center of the Gulf, with 135 mg O2 m−2 h−1 in spring (3.2 g O2 m−2 d−1); in the other locations, it ranged from 3.3 to 58.2 mg O2 m−2 h−1 (0.08–1.4 g O2 m−2 d−1). Compared to phytoplankton, microphytobenthos production was higher only in the bottoms < 1 m depth. In deeper bottom waters, phytoplankton production could be absent due to light limitation, while microphytobenthos was still productive. Phytoplankton production m−2 was generally higher than microphytobenthic production. Microphytobenthic biomass, higher than phytoplanktonic, varied from 27 to 379 mg Chl a m−2, the maximum in the mussel farm sediments, with the minimum in sandy shallow bottoms. Pigment analysis showed that microphytobenthos consisted mainly of diatoms (Chl c and fucoxanthin) but other algal groups containing Chl b could become seasonally important. A Principal Component Analysis suggested that the main statistical factors explaining the distribution of our observations may be interpreted in terms of enrichment in phaeopigments and light; the role of Chl a appearing paradoxically as secondary in benthic production rates. Phaeopigments are mainly constituted by phaeophorbides, which indicate grazing processes. The influence of the mussel farm on the oxygen balance is noticeable in the whole Gulf.