The microphytobenthos consists of unicellular eukaryotic algae and cyanobacteria that grow within the upper several millimeters of illuminated sediments, typically appearing only as a subtle brownish or greenish shading. The surficial layer of the sediment is a zone of intense microbial and geochemical activity and of considerable physical reworking. In many shallow ecosystems, the biomass of benthic microalgae often exceeds that of the phytoplankton in the overlying waters. Direct comparison of the abundance of benthic and suspended microalgae is complicated by the means used to measure biomass and by the vertical and horizontal distribution of the microphytobenthos in the sediment. Where biomass has been estimated as chlorophyll a, there may be negligible to large (40%) error due to interference by degradation products, except where chlorophyll is measured by high-performance liquid chromatography. The vertical distribution of microphytobenthos, aside from mat-forming species, is determined by the opposing effects of their vertical migration, which tends to concentrate them near the surface, and physical mixing by overlying currents, which tends to cause an even vertical distribution through the mixed layer of sediment. Uncertainties in vertical distribution are compounded by frequently patchy horizontal distribution. Under-sampling on small (<1 m) scales can lead to errors in the estimate that are comparable to the ranges of seasonal and geographic variation. These uncertainties are compounded by biases in the techniques used to estimate production by the microphytobenthos. In most environments studied, biomass (as chlorophyll a) and light availability appear to be the principal determinants of benthic primary production. The effect of variable light intensities on integral production can be described by a functional response curve. When normalized to the chlorophyll content of the surficial sediment, the residual variation in the data described by the functional response curve is due to changes in the chlorophyll-specific response to irradiance. Production by the benthos is often a significant fraction of production in the water column and microphytobenthos may contribute directly to water column production when they are resuspended. Thus on both the basis of biomass and biogeochemical reactivity, benthic microalgae play significant roles in system productivity and trophic dynamics, as well as such habitat characteristics as sediment stability. *** DIRECT SUPPORT *** A01BY074 00003

https://www.jstor.org/stable/info/1352224

Microphytobenthos: The ecological role of the “secret garden” of unvegetated, shallow-water marine habitats. I. Distribution, abundance and primary production

Assessing the trophic state and eutrophication of coastal marine systems: a new approach based on the biochemical composition of sediment organic matter

The sandy sediments that blanket the inner shelf are situated in a zone where nutrient input from land and strong mixing produce maximum primary production and tight coupling between water column and sedimentary processes. The high permeability of the shelf sands renders them susceptible to pressure gradients generated by hydrodynamic and biological forces that modulate spatial and temporal patterns of water circulation through these sediments. The resulting dynamic three-dimensional patterns of particle and solute distribution generate a broad spectrum of biogeochemical reaction zones that facilitate effective decomposition of the pelagic and benthic primary production products. The intricate coupling between the water column and sediment makes it challenging to quantify the production and decomposition processes and the resultant fluxes in permeable shelf sands. Recent technical developments have led to insights into the high biogeochemical and biological activity of these permeable sediments and their role in the global cycles of matter.

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Benthic Exchange and Biogeochemical Cycling in Permeable Sediments

—With the passage of the Sustainable Fisheries Act in the fall of 1996, significant new opportunities and challenges exist in the United States to protect and conserve the habitat of marine, estuarine, and anadromous finfish as well as key populations of mollusks and crustaceans. As of October 1998, all federal fishery management councils (the Councils) were required to amend their fishery management plans (covering over 700 stocks) to identify, for each species, the essential fish habitat, which is “those waters and substrate necessary to fish for spawning, feeding or growth to maturity.” Threats to habitat and steps necessary to ameliorate those threats also had to be identified. Information from fisheries scientists and managers throughout the country will be needed to accurately identify essential fish habitat and habitat threats and to monitor the effectiveness of protective measures that come into force once habitat has been identified as essential. My vision, which is also that of the National Oceanic and Atmospheric Administration, is no further loss of habitat quantity and quality as well as the preservation and restoration of habitat biodiversity by the year 2004. The essential fish habitat provisions of the Magnuson-Stevens Act provide an outstanding opportunity for the National Marine Fisheries Service, the Councils, and our numerous partners from every sector of society to develop new ecosystem approaches to fishery management addressing cumulative impacts to habitats in a comprehensive, effective, and efficient manner.

Fish habitat: essential fish habitat and rehabilitation

In situ, paired light and dark benthic flux chamber incubations were used to estimate the exchange of nutrients, oxygen and inorganic carbon across the sediment – water interface of the South Atlantic Bight (SAB) continental shelf. The results indicate that physically forced non-diffusive pore water transport and benthic primary production (BPP) by sea floor microalgae exert a major influence on benthic exchange rates on the mid- and outer-continental shelf (depths of 14–40 m). Light fluxes to the sea floor and sediment photosynthetic pigment distributions determined on two, widely spaced cross-shelf transects suggest that BPP may occur over 84% of the SAB continental shelf area. Microalgal gross BPP rates at all study sites averaged 400±260 mg C m−2 d−1 between May and September 1996 while water column primary productivity averaged 682±176 mg C m−2 d−1, implying a total primary productivity for this region of approximately 1100 mg C m−2 d−1 (1.6 times the water column productivity alone). The results are also consistent with the advective transport of pore waters. Benthic flux chambers appear to retard this exchange, affecting the accuracy of derived net fluxes. Given our inability to relate pore water gradients to fluxes in non-diffusive regimes and to mimic natural advective transport in intact core incubations, traditional techniques such as pore water gradient diffusion calculations or shipboard core incubations also may not provide accurate flux estimates. Because of these limitations, fundamental questions remain concerning the processes that control nutrient inventories in pore waters and the magnitude of the net benthic flux of nutrients on the sandy SAB shelf.

Benthic flux of biogenic elements on the Southeastern US continental shelf: influence of pore water advective transport and benthic microalgae

Benthic microalgal production and sediment respiration were measured in si tu at 8 locations in Onslow Bay, a portion of the North Carolina continental shelf, between 1984 and 1989, by measuring changes in dissolved oxygen concentrations inside clear and opaque benthic chambers placed by divers. Measurements were also made of water column productivity, phytoplankton biomass, benthic microalgal biomass, light flux, and temperature. Benthic production and respiration were measured from March through October at locations ranging from 14.6 to 41 m deep. Gross benthic microalgal production averaged 24.9 mg C m'h' , compared to average integrated water column production of 27.4 mg C m-' h' Sediment respiration rates averaged 18.1 mg C m-' h' . Benthic microalgal biomass averaged 36.4 mg chl a m-2, and always exceeded integrated phytoplankton biomass, which averaged 8.2 mg chl a m-'. Benthic mlcroalgae had much lower average assimilation numbers than phytoplankton (0.8 vs 5.2 mg C (mg chl a) ' h' ) . Benthic microalgae make a significant contribution to total shelf production in Onslow Bay Open sand bottom habitats in this ecosystem must now be viewed as productive, rather than relatively barren.

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LB Cahoon

Papers

Benthic microalgal production in Onslow Bay, North Carolina, USA