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  1. Abstract

    Megaripples are current‐generated seafloor bedforms of well‐sorted sand or gravel and wavelengths over 1 m. In this aquatic eddy covariance study, we measured large rates of benthic primary production and respiration for a shallow‐water sandy megaripple field exposed to strong tidally driven currents and intense sunlight. Current and light were the main short‐term drivers of a highly dynamic oxygen exchange. Daytime oxygen release as high as 300 mmol m−2 d−1and nighttime oxygen uptake up to −100 mmol m−2 d−1were likely sustained by current‐driven transport of oxygen, nutrients, and organic matter (fuel) into and out of the sand and superimposed by rapid internal cycling. Seasonal differences in temperature (45%) and light (69%) between April and September were the main long‐term drivers of substantially greater rates of gross primary production and respiration in September. The megaripples functioned as an intense metabolic hotspot with carbon cycling rates larger than those of most near‐shore sediments.

  2. Abstract

    The aquatic eddy covariance technique is increasingly used to determine oxygen (O2) fluxes over benthic ecosystems. The technique uses O2measuring systems that have a high temporal and numerical resolution. In this study, we performed a series of lab and field tests to assess a new optical submersible O2meter designed for aquatic eddy covariance measurements and equipped with an existing ultra‐high speed optical fiber sensor. The meter has a 16‐bit digital‐to‐analog‐signal conversion that produces a 0–5 V output at a rate up to 40 Hz. The device was paired with an acoustic Doppler velocimeter. The combined meter and fiber‐optic O2sensor's response time was significantly faster in O2‐undersaturated water compared to in O2‐supersaturated water (0.087 vs. 0.12 s), but still sufficiently fast for aquatic eddy covariance measurements. The O2optode signal was not sensitive to variations in water flow or light exposure. However, the response time was affected by the direction of the flow. When the sensor tip was exposed to a flow from the back rather than the front, the response time increased by 37%. The meter's internal signal processing time was determined to be ~ 0.05 s, a delay that can be corrected for during postprocessing. In order for the built‐in temperature correction to be accurate,more »the meter should always be submerged with the fiber‐optic sensor. In multiple 21–47 h field tests, the system recorded consistently high‐quality, low‐noise O2flux data. Overall, the new meter is a powerful option for collecting robust aquatic eddy covariance data.

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  3. Abstract Many coastal ecosystems, such as coral reefs and seagrass meadows, currently experience overgrowth by fleshy algae due to the interplay of local and global stressors. This is usually accompanied by strong decreases in habitat complexity and biodiversity. Recently, persistent, mat-forming fleshy red algae, previously described for the Black Sea and several Atlantic locations, have also been observed in the Mediterranean. These several centimetre high mats may displace seagrass meadows and invertebrate communities, potentially causing a substantial loss of associated biodiversity. We show that the sessile invertebrate biodiversity in these red algae mats is high and exceeds that of neighbouring seagrass meadows. Comparative biodiversity indices were similar to or higher than those recently described for calcifying green algae habitats and biodiversity hotspots like coral reefs or mangrove forests. Our findings suggest that fleshy red algae mats can act as alternative habitats and temporary sessile invertebrate biodiversity reservoirs in times of environmental change.
    Free, publicly-accessible full text available December 1, 2023
  4. Aquatic eddy covariance (AEC) is increasingly being used to study benthic oxygen (O 2 ) flux dynamics, organic carbon cycling, and ecosystem health in marine and freshwater environments. Because it is a noninvasive technique, has a high temporal resolution (∼15 min), and integrates over a large area of the seafloor (typically 10–100 m 2 ), it has provided new insights on the functioning of aquatic ecosystems under naturally varying in situ conditions and has given us more accurate assessments of their metabolism. In this review, we summarize biogeochemical, ecological, and biological insightsgained from AEC studies of marine ecosystems. A general finding for all substrates is that benthic O 2 exchange is far more dynamic than earlier recognized, and thus accurate mean values can only be obtained from measurements that integrate over all timescales that affect the local O 2 exchange. Finally, we highlight new developments of the technique, including measurements of air–water gas exchange and long-term deployments.
  5. Abstract. The aquatic eddy covariance technique stands out as a powerful method for benthic O2 flux measurements in shelf environments because itintegrates effects of naturally varying drivers of the flux such as current flow and light. In conventional eddy covariance instruments, the timeshift caused by spatial separation of the measuring locations of flow and O2 concentration can produce substantial flux errors that aredifficult to correct. We here introduce a triple O2 sensor eddy covariance instrument (3OEC) that by instrument design eliminates theseerrors. This is achieved by positioning three O2 sensors around the flow measuring volume, which allows the O2concentration to be calculated at the point of the current flow measurements. The new instrument was tested in an energetic coastal environment with highly permeablecoral reef sands colonised by microphytobenthos. Parallel deployments of the 3OEC and a conventional eddy covariance system (2OEC) demonstrate thatthe new instrument produces more consistent fluxes with lower error margin. 3OEC fluxes in general were lower than 2OEC fluxes, and the nighttimefluxes recorded by the two instruments were statistically different. We attribute this to the elimination of uncertainties associated with the timeshift correction. The deployments at ∼ 10 m water depth revealed high day- and nighttime O2 fluxes despite themore »relatively loworganic content of the coarse sediment and overlying water. High light utilisation efficiency of the microphytobenthos and bottom currents increasingpore water exchange facilitated the high benthic production and coupled respiration. 3OEC measurements after sunset documented a gradual transfer ofnegative flux signals from the small turbulence generated at the sediment–water interface to the larger wave-dominated eddies of the overlying watercolumn that still carried a positive flux signal, suggesting concurrent fluxes in opposite directions depending on eddy size and a memory effect oflarge eddies. The results demonstrate that the 3OEC can improve the precision of benthic flux measurements, including measurements in environmentsconsidered challenging for the eddy covariance technique, and thereby produce novel insights into the mechanisms that control flux. We consider thefluxes produced by this instrument for the permeable reef sands the most realistic achievable with present-day technology.« less
  6. Abstract. Sediment–water oxygen fluxes are widely used as a proxy fororganic carbon production and mineralization at the seafloor. In situ fluxescan be measured non-invasively with the aquatic eddy covariance technique,but a critical requirement is that the sensors of the instrument are able tocorrectly capture the high-frequency variations in dissolved oxygenconcentration and vertical velocity. Even small changes in sensorcharacteristics during deployment as caused, e.g. by biofouling can result inerroneous flux data. Here we present a dual-optode eddy covarianceinstrument (2OEC) with two fast oxygen fibre sensors and document howerroneous flux interpretations and data loss can effectively be reduced bythis hardware and a new data analysis approach. With deployments over acarbonate sandy sediment in the Florida Keys and comparison with parallelbenthic advection chamber incubations, we demonstrate the improved dataquality and data reliability facilitated by the instrument and associateddata processing. Short-term changes in flux that are dubious in measurementswith single oxygen sensor instruments can be confirmed or rejected with the2OEC and in our deployments provided new insights into the temporal dynamicsof benthic oxygen flux in permeable carbonate sands. Under steadyconditions, representative benthic flux data can be generated with the 2OECwithin a couple of hours, making this technique suitable for mappingsediment–water, intra-water column, or atmosphere–watermore »fluxes.« less
  7. Abstract

    Sediment-oil-agglomerates (SOA) are one of the most common forms of contamination impacting shores after a major oil spill; and following the Deepwater Horizon (DWH) accident, large numbers of SOAs were buried in the sandy beaches of the northeastern Gulf of Mexico. SOAs provide a source of toxic oil compounds, and although SOAs can persist for many years, their long-term fate was unknown. Here we report the results of a 3-yearin-situexperiment that quantified the degradation of standardized SOAs buried in the upper 50 cm of a North Florida sandy beach. Time series of hydrocarbon mass, carbon content, n-alkanes, PAHs, and fluorescence indicate that the decomposition of golf-ball-size DWH-SOAs embedded in beach sand takes at least 32 years, while SOA degradation without sediment contact would require more than 100 years. SOA alkane and PAH decay rates within the sediment were similar to those at the beach surface. The porous structure of the SOAs kept their cores oxygen-replete. The results reveal that SOAs buried deep in beach sands can be decomposed through relatively rapid aerobic microbial oil degradation in the tidally ventilated permeable beach sand, emphasizing the role of the sandy beach as an aerobic biocatalytical reactor at the land-ocean interface.