Benthic Microalgal Production Research Articles

Glacial melt disturbance shifts community metabolism of an Antarctic seafloor ecosystem from net autotrophy to heterotrophy

Climate change-induced glacial melt affects benthic ecosystems along the West Antarctic Peninsula, but current understanding of the effects on benthic primary production and respiration is limited. Here we demonstrate with a series of in situ community metabolism measurements that climate-related glacial melt disturbance shifts benthic communities from net autotrophy to heterotrophy. With little glacial melt disturbance (during cold El Niño spring 2015), clear waters enabled high benthic microalgal production, resulting in net autotrophic benthic communities. In contrast, water column turbidity caused by increased glacial melt run-off (summer 2015 and warm La Niña spring 2016) limited benthic microalgal production and turned the benthic communities net heterotrophic. Ongoing accelerations in glacial melt and run-off may steer shallow Antarctic seafloor ecosystems towards net heterotrophy, altering the metabolic balance of benthic communities and potentially impacting the carbon balance and food webs at the Antarctic seafloor.

Nitrogen Cycling and Mass Balance in the World’s Mangrove Forests

Nitrogen (N) cycling in mangroves is complex, with rapid turnover of low dissolved N concentrations, but slow turnover of particulate N. Most N is stored in soils. The largest sources of N are nearly equal amounts of mangrove and benthic microalgal primary production. Dissolved N fluxes between the forests and tidal waters show net uptake, indicating N conservation. N2-fixation is underestimated as rapid rates measured on tree stems, aboveground roots and cyanobacterial mats cannot currently be accounted for at the whole-forest scale due to their extreme patchiness and the inability to extrapolate beyond a localized area. Net immobilization of NH4+ is the largest ecosystem flux, indicating N retention. Denitrification is the largest loss of N, equating to 35% of total N input. Burial equates to about 29% of total inputs and is the second largest loss of N. Total inputs slightly exceed total outputs, currently suggesting net N balance in mangroves. Mangrove PON export equates to ≈95% of PON export from the world’s tropical rivers, but only 1.5% of the entire world’s river discharge. Mangrove N2O emissions, denitrification, and burial contribute 0.4%, 0.5–2.0% and 6%, respectively, to the global coastal ocean, which are disproportionate to their small worldwide area.

Recovery of the salt marsh periwinkle (Littoraria irrorata) 9 years after the Deepwater Horizon oil spill: Size matters

Prior studies indicated salt marsh periwinkles (Littoraria irrorata) were strongly impacted in heavily oiled marshes for at least 5 years following the Deepwater Horizon oil spill. Here, we detail longer-term effects and recovery over nine years. Our analysis found that neither density nor population size structure recovered at heavily oiled sites where snails were smaller and variability in size structure and density was increased. Total aboveground live plant biomass and stem density remained lower over time in heavily oiled marshes, and we speculate that the resulting more open canopy stimulated benthic microalgal production contributing to high spring periwinkle densities or that the lower stem density reduced the ability of subadults and small adults to escape predation. Our data indicate that periwinkle population recovery may take one to two decades after the oil spill at moderately oiled and heavily oiled sites, respectively.

Taiwanese marine microbenthic algal communities remain similar yet chlorophyll a concentrations rise in mesocosms with elevated CO2 and temperature

The effects of increasing CO2 concentrations and temperature on microalgal assemblages were examined in Taiwan using mesocosms that simulate coral reef ecosystem. We assessed changes in abundance and diversity of benthic algae grown at 25°C and 28°C, under ambient (~400μatm) and at high CO2 conditions (800–1000μatm). Total alkalinity, pCO2, and the aragonite saturation state, were all significantly different between control and high CO2 treatments in both temperature treatments. Chl a concentration increased significantly in CO2-treated groups at 25°C, but benthic microalgal abundance was not significantly different. The number of microalgal species and the microalgal community structure did not differ between control and CO2-treated groups at both temperatures. Our results suggest that increasing CO2 may boost benthic microalgal primary productivity if sufficient nutrients are available, although site-specific responses are difficult to predict.

Importance of coastal primary production in the northern Baltic Sea.

In this study, we measured depth-dependent benthic microalgal primary production in a Bothnian Bay estuary to estimate the benthic contribution to total primary production. In addition, we compiled data on benthic microalgal primary production in the entire Baltic Sea. In the estuary, the benthic habitat contributed 17 % to the total annual primary production, and when upscaling our data to the entire Bothnian Bay, the corresponding value was 31 %. This estimated benthic share (31 %) is three times higher compared to past estimates of 10 %. The main reason for this discrepancy is the lack of data regarding benthic primary production in the northern Baltic Sea, but also that past studies overestimated the importance of pelagic primary production by not correcting for system-specific bathymetric variation. Our study thus highlights the importance of benthic communities for the northern Baltic Sea ecosystem in general and for future management strategies and ecosystem studies in particular.Electronic supplementary materialThe online version of this article (doi:10.1007/s13280-016-0778-5) contains supplementary material, which is available to authorized users.

Nutrient Fluxes from Sediments in the San Francisco Bay Delta

In September 2011 and March 2012, benthic nutrient fluxes were measured in the San Francisco Bay Delta, across a gradient from above the confluence of the Sacramento and San Joaquin Rivers to Suisun Bay. Dark and illuminated core incubation techniques were used to measure rates of denitrification, nutrient fluxes (phosphate, ammonium, nitrate), and oxygen fluxes. While benthic nutrient fluxes have been assessed at several sites in northern San Francisco Bay, such data across a Delta–Bay transect have not previously been determined. Average September rates of DIN (nitrate, nitrite, ammonium) flux were net positive across all sites, while March DIN flux indicated net uptake of DIN at some sites. Denitrification rates based on the N2/Ar ratio approach were between 0.6 and 1.0 mmol m−2 day−1, similar to other mesotrophic estuarine sediments. Coupled nitrification–denitrification was the dominant denitrification pathway in September, with higher overlying water nitrate concentrations in March resulting in denitrification driven by nitrate flux into the sediments. Estimated benthic microalgal productivity was variable and surprisingly high in Delta sediments and may represent a major source of labile carbon to this ecosystem. Variable N/P stoichiometry was observed in these sediments, with deviations from Redfield driven by processes such as denitrification, variable light/dark uptake of nutrients by microalgae, and adsorption of soluble reactive phosphorus.

Threat of predation alters the ability of benthic invertebrates to modify sediment biogeochemistry and benthic microalgal abundance

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 494:29-39 (2013) - DOI: https://doi.org/10.3354/meps10561 Threat of predation alters the ability of benthic invertebrates to modify sediment biogeochemistry and benthic microalgal abundance Katherine M. Premo, Anna Christina Tyler* Program in Environmental Sciences, School of Life Sciences, Rochester Institute of Technology, 85 Lomb Memorial Drive, Rochester, New York 14623, USA *Corresponding author. Email: actsbi@rit.edu ABSTRACT: Benthic invertebrates alter estuarine ecosystem function by moderating benthic microalgal production and sediment biogeochemistry. While lethal predation eliminates the effect of invertebrates, non-consumptive effects of predation through trait-mediated indirect effects on behavior may reduce prey control of ecosystem function. Using microcosms, we investigated how chemical cues from the predatory mud crab Panopeus herbstii changed prey behavior and thereby affected the ability of the grazing-deposit feeding gastropod Ilyanassa obsoleta and filter-feeding bivalve Mercenaria mercenaria to regulate benthic ecosystem function. Sediment-water column fluxes of oxygen and nutrients, microscale profiles of sediment oxygen, benthic chlorophyll a, porewater ammonium and organic matter were measured after exposure to predator effluent. Both species had significant effects on sediment processes, with higher sediment oxygen consumption and release of ammonium to the water column, and lower porewater ammonium. However, individual effects were altered in the presence of chemical cues from P. herbstii. The negative effect of M. mercenaria on porewater ammonium was diminished by 26% in predator treatments, suggesting less movement and therefore less sediment oxidation and nitrogen removal. For I. obsoleta, there was a 30% increase in microalgal biomass and a 35% decrease in sediment ammonium release in the predator treatments, possibly associated with decreased foraging activity. In these experiments, non-consumptive predator effects indirectly altered benthic biogeochemistry, benthic microalgae and nutrient fluxes. These results suggest that experiments conducted without predators may overestimate the impact of benthic fauna on ecosystem processes and that changes in community structure resulting in loss of predators may have indirect effects on benthic ecosystem function, leading to faster nitrogen turnover and release to the water column. KEY WORDS: Trait-mediated interactions · Non-consumptive effects · Benthic invertebrates · Benthic microalgae · Sediment biogeochemistry Full text in pdf format PreviousNextCite this article as: Premo KM, Tyler AC (2013) Threat of predation alters the ability of benthic invertebrates to modify sediment biogeochemistry and benthic microalgal abundance. Mar Ecol Prog Ser 494:29-39. https://doi.org/10.3354/meps10561 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 494. Online publication date: December 04, 2013 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2013 Inter-Research.

Consumers mediate the effects of experimental ocean acidification and warming on primary producers

It is well known that ocean acidification can have profound impacts on marine organisms. However, we know little about the direct and indirect effects of ocean acidification and also how these effects interact with other features of environmental change such as warming and declining consumer pressure. In this study, we tested whether the presence of consumers (invertebrate mesograzers) influenced the interactive effects of ocean acidification and warming on benthic microalgae in a seagrass community mesocosm experiment. Net effects of acidification and warming on benthic microalgal biomass and production, as assessed by analysis of variance, were relatively weak regardless of grazer presence. However, partitioning these net effects into direct and indirect effects using structural equation modeling revealed several strong relationships. In the absence of grazers, benthic microalgae were negatively and indirectly affected by sediment-associated microalgal grazers and macroalgal shading, but directly and positively affected by acidification and warming. Combining indirect and direct effects yielded no or weak net effects. In the presence of grazers, almost all direct and indirect climate effects were nonsignificant. Our analyses highlight that (i) indirect effects of climate change may be at least as strong as direct effects, (ii) grazers are crucial in mediating these effects, and (iii) effects of ocean acidification may be apparent only through indirect effects and in combination with other variables (e.g., warming). These findings highlight the importance of experimental designs and statistical analyses that allow us to separate and quantify the direct and indirect effects of multiple climate variables on natural communities.

Sand Bottom Microalgal Production and Benthic Nutrient Fluxes on the Northeastern Gulf of Mexico Nearshore Shelf

Benthic microalgal production on the continental shelves may be an important contributor to the overall productivity of offshore ecosystems. We used light and dark benthic chambers to measure in situ production, respiration, and benthic nutrient flux on the nearshore quartzite sands of the northeast Gulf of Mexico shelf. Net exchange of O2, NH4+, NO3- + NO2-, PO4-3, and SiO2 was measured in samples taken from chambers at depths of 15 to 16 m offshore of Pensacola, FL. Phytoplankton production and respiration in near-bottom water was determined in paired light/dark BOD bottles to correct chamber measurements for water column processes. Sediment chlorophyll a (Chl a) averaged 4.8 μg Chl a/g. Phytoplankton averaged 5.5 μg Chl a/L. Pheophytin:chlorophyll ratios for the sediment were near 1 indicating an actively growing algal community. Phytoplankton net production ranged from 0.6 to 2.8 mg C/m3/hr. Benthic net production in three separate determinations was 17.7 ± 6.1, 9.5 ± 2.9, and 8.8 ± 1.6 mg C/m2/hr. Benthic respiration was 24.8 ± 0.7, 30.8 ± 1.4, and 11.3 ± 0.3 mg C/m2/hr, respectively. Benthic gross production was thus 42.5 ± 5.2, 40.3 ± 1.2, and 20.3 ± 1.7 mg C/m2/hr, respectively. Benthic nutrient fluxes were highly variable and generally low. Sediment uptake was observed for NH4+ and PO4-3 throughout the study. NO3- + NO2- and SiO2 uptake was observed in 2004 with sediment release seen in 2005.

Benthic microalgal production in the Arctic: applied methods and status of the current database

Abstract The current database on benthic microalgal production in Arctic waters comprises 10 peer-reviewed and three unpublished studies. Here, we compile and discuss these datasets, along with the applied measurement approaches used. The latter is essential for robust comparative analysis and to clarify the often very confusing terminology in the existing literature. Our compilation demonstrates that i) benthic microalgae contribute significantly to coastal ecosystem production in the Arctic, and ii) benthic microalgal production on average exceeds pelagic productivity by a factor of 1.5 for water depths down to 30 m. We have established relationships between irradiance, water depth and benthic microalgal productivity that can be used to extrapolate results from quantitative experimental studies to the entire Arctic region. Two different approaches estimated that current benthic microalgal production in the Arctic is between 1.1 and 1.6×107 tons C year-1. Climate change is expected to increase the overall primary production and affect the balance between pelagic and benthic productivity in the Arctic. It is therefore imperative to get better quantitative understanding of the relationship between increased freshwater run-off, shrinking sea-ice cover, light availability and benthic primary production to assess future impact on the Arctic food web and trophic coupling.

Stable isotopes reveal life history polymorphism in the coastal fish Apogon notatus

In the coastal marine fish Apogon notatus most individuals emigrate from the neritic breeding ground in winter and return there in the following spring (migratory type), while some indi- viduals reside in the neritic habitat throughout the year (resident type). In the present study, we con- ducted carbon and nitrogen stable isotope analysis to estimate their winter migration routes and to elucidate the underlying spatial heterogeneity in the food web structure of the coastal ecosystem, and compared the results with those of A. semilineatus, a deepwater congener. A. notatus fed on a vari- ety of zooplankton and zoobenthos in the neritic habitat, whereas A. semilineatus fed on pelagic mysids exclusively. The former showed more enriched δ 13 C than did the latter, indicating strong reliance on benthic production. Such a difference in stable isotope ratio was attributed to spatial heterogeneity in the coastal food web structure: benthic microalgal production predominates in the neritic food web, while the pelagic deepwater food web is characterized by phytoplankton produc- tion. After returning from winter migration, A. notatus showed marked depletion of δ 13 C compared to its non-migratory counterpart residing in the neritic habitat. Thereafter, the migratory type showed enrichment of δ 13 C during its stay in the neritic breeding ground, approaching that of the resident type. The results suggest that most A. notatus migrated to the deepwater habitat and spent a couple of winter months there. Stable isotope analysis revealed that life history polymorphism (i.e. migratory and resident types) exists in the A. notatus population.

The recovery of microalgal production and biomass in a South African temporarily open/closed estuary, following mouth breaching

Mouth breaching is a recurrent event in temporarily open/closed estuaries (TOCEs). Such disturbances result in flushing and sediment scouring, reducing the microalgal biomass stock. The depletion of these microalgae may have negative repercussions in the form of depleted stocks of commercial fish, game fish, crustaceans and mollusks. The aim of this investigation was therefore: (1) to monitor the recovery of microalgal biomass and production following a breaching event; and (2) to determine the key environmental parameters influencing primary production during the open and recovery phases. Phytoplankton and benthic microalgal production was measured ( 14C-uptake method) successively during the closed, open and recovery phases of the Mdloti TOCE (South Africa). Upon breaching, 94–99% of microalgal biomass was washed out to sea through flushing and sediment scouring. A temporary recovery of phytoplankton and benthic microalgal biomass was observed during the open phase, but this was not sustained because of continual flushing and scouring of the sediment. During the re-closure (recovery phase), microalgal biomass immediately increased, reaching pre-breaching levels 35–40 days following the breaching event. In contrast to biomass, autochthonous pelagic primary production reached a maximum level (341 mg C m −2 h −1) during the open phase. Pelagic primary production normalized to biomass (P B) significantly increased during the open phase. This is attributed to a favorable combination of optimum light conditions, high influx of macronutrients and high water temperatures (33 °C). Similarly, benthic primary production normalized to biomass (P B) peaked during the open phase (35 mg C mg chl- a −1 h −1). Multivariate analysis showed that major variations in primary production were mainly controlled by temperature, dissolved inorganic nitrogen (DIN) to phosphorus (DIP) molar ratios (water-column and pore-water) and light extinction ( K d), all of which were regulated by the state of the mouth.

Live benthic diatoms from the upper continental slope: extending the limits of marine primary production

Discovery of obligate benthic diatoms living as deep as 191 m substantially extends the known depth range of these primary producers and holds significant implications for oceanic productivity and biogeochemical cycling. Species of the epipsammic, monoraphid genus Cocconeis dominated the ≥35 species of living benthic diatoms identified from the North Carolina continental margin in samples collected at bottom depths from 67 to 191 m. A total of 126 species were identified from prepared samples, more than 90% of which are obligate benthic forms. Mid-day, near bottom, photosynthetically active radiation values recorded at the 191 m site averaged 0.106 µmol photons m -2 s -1 , representing about 0.028% of surface incident radiation and resulting from a water column attenuation coefficient of 0.0446 m -1 . The presence of active benthic microalgae in these extremely low light conditions suggests the development of special light-harvesting adaptations including elevated levels of the blue-light absorbing accessory pigment, fucoxanthin. Extending the limit of benthic microalgal production to upper slope depths offshore from North Carolina increases the estimated total benthic primary production in that area of the continental margin by about 14%. At present, extrapolating potential increases in benthic microalgal biomass and production resulting from extending the depth limits of viable benthic microalgae to a larger, global, oceanic scale is limited by paucity of data.

Impact of canal development on intertidal microalgal productivity: Comparative assessment of Patterson Lakes and Ralphs Bay, South East Australia

We assessed the potential impact of a proposed canal development in an estuarine sandflat at Ralphs Bay, Tasmania on intertidal microalgal productivity and species composition, by comparing it over summer and winter seasons with a well- established (30 year old) canal estate at Patterson Lakes, Victoria. Pulse amplitude modulation (PAM) fluorometry was used to generate a relative measure of photosynthetic performance, which combined with microalgal chlorophyll biomass and irradiance provides an assessment of potential primary productivity. We present a sophisticated mathematical model for calculating benthic microalgal production and the contribution to total primary production, taking into account sediment light attenuation as estimated from sediment grain size. Ralphs Bay had a total productive microalgal biomass of 44 mg chlorophyll a m−2 which was six times higher than Patterson Lakes, while the relative productivity of Ralphs Bay was four times greater compared to Patterson Lakes where productivity was virtually absent in the subtidal zone of the canal waterway. Ralphs Bay exhibited a more or less homogeneous spatial distribution of microphytobenthos biomass but this was subject to some seasonal variation in species composition, abundance and productivity. By contrast, at Patterson Lakes biomass distribution, diversity and productivity was highly spatially variable in the canal system in both seasons. Patterson Lakes exhibited 60% lower microphytobenthos species richness than Ralphs Bay but little variation in species composition occurred between seasons in the canal estate. This suggests that the dominant diatom species in Patterson Lakes, Pinnularia yarrensis, Gyrosigma balticum and Pleurosigma salinarum, are well adapted to the disturbance regime within the canal estate. The proposed canal development at Ralphs Bay is estimated to cause a decrease in microalgal productivity by both reducing available marine substrate (66% reduction) and replacing productive intertidal phytobenthic habitat with nonproductive canal substrate. These combine to cause a decline in productivity of 92% with significant flow-on effects predicted for higher trophic levels such as migratory wading birds.

Food web analysis of an eelgrass ( Zostera marina L.) meadow and neighbouring sites in Mitsukuchi Bay (Seto Inland Sea, Japan) using carbon and nitrogen stable isotope ratios

The contribution of benthic microalgal production has been compared both within and outside a coastal eelgrass ( Zostera marina L.) meadow. Carbon and nitrogen stable isotope ratios of suspended particulate organic matter (POM), epiphytic and epilithic organic matter (EOM), leaves of Z. marina (inside the meadow only) and two secondary consumer species (small crustaceans and fish) were measured inside and outside a meadow in Mitsukuchi Bay, Northwest Seto Inland Sea, Japan. Inside the meadow, primary producers (epiphyton) and consumers showed higher δ 13C signatures than outside. Primary and secondary consumers inside the meadow were mainly dependent on epiphyton on the leaves of Z. marina, while consumer species outside the meadow were basically dependent on epilithon.

Nitrogen cycling and ecosystem exchanges in a Virginia tidal freshwater marsh

Tidal freshwater marshes are diverse habitats that differ both within and between marshes in terms of plant community composition, sediment type, marsh elevation, and nutrient status. Because our knowledge of the nitrogen (N) biogeochemistry of tidal freshwater systems is limited, it is difficult to assess how these marshes will respond to long-term progressive nutrient loading due to watershed development and urbanization. We present a process-based mass balance model of N cycling in Sweet Hall marsh, a pristine (i.e., low nutrient)Peltandra virginica-Pontederia cordata dominated tidal freshwater marsh in the York River estuary, Virginia. The model, which was based on a combination of field and literature data, revealed that N cycling in the system was largely conservative. The mineralization of organic N to NH4 + provided almost twice as much inorganic N as was needed to support marsh macrophyte and benthic microalgal primary production. Efficient utilization of porewater NH4 + by nitrifiers and other microbes resulted in low rates of tidal NH4 + export from the marsh and little accumulation of NH4 + in marsh porewaters. Inputs of N from the estuary and atmosphere were not critical in supporting marsh primary production, and served to balance N losses due to denitrification and burial. A comparison of these results with the literature suggests that the relative importance of tidal freshwater marsh N cycling processes, including plant productivity, organic matter mineralization, microbial immobilization, and coupled nitrification-denitrification, are largely independent of small changes in water column N loading. Although very high (millimolar) concentrations of dissolved inorganic N can affect processes including denitrification and plant productivity, the factors that cause the switch from efficient N recycling to a more open N cycle have not yet been identified.

Factors influencing nutrient dynamics in the eutrophic Jiaozhou Bay, North China

We conducted studies of nutrients and water mass movements in a semi-enclosed bay in northern China to understand nutrient dynamics under varying tidal regimes. Four cruises were conducted under varying tidal regimes in Jiaozhou Bay, two at neap tide and one at spring tide in August and one at spring tide in October 2001. In addition to transect surveys, drift experiments and an anchor station were employed to show current and tidal effects. Samples for nutrient evaluation were taken from the five major tributary rivers in March (dry season) and August (flood season) of 2002 to estimate nutrient transport by rivers, and wastewater samples were collected to evaluate nutrients in wastewater discharge. Benthic nutrient fluxes were determined by (1) incubation of sediments with overlying seawater on board the boat and (2) calculated by Fick’s First Law from nutrient pore water profiles. Nutrient concentrations were high in the north, especially the northeast and northwest sectors, reflecting human activities. Jiaozhou Bay was characterized by high nitrogen, but low phosphorus and silica concentrations compared to Chinese coastal seas. Based on nutrient atomic ratios, the limiting elements for phytoplankton growth in Jiaozhou Bay were silica and phosphorus. The fluxes of nutrients between sediment and overlying water varied depending on the specific nutrient, the site and redox conditions. Benthic nutrient fluxes based on sediment incubations were all lower than the estimated diffusive fluxes, implying that the nutrients released from sediment pore waters were probably utilized by benthic microalgal and bottom-water primary production. A preliminary estimate of nutrient budgets demonstrated that riverine and wastewater inputs were greater than atmospheric deposition into Jiaozhou Bay, except that nitrate from wastewater inputs was less than atmospheric deposition. Concentrations of nitrogen and phosphorus increased while silica decreased in the last four decades, similar to other eutrophicated estuaries. The resulting shift in nutrient composition in Jiaozhou Bay affects phytoplankton composition, trophic interactions, and sustainability of living resources.

Contrasting food-web support bases for adjoining river-influenced and non-river influenced continental shelf ecosystems

Nutrient and chlorophyll a concentrations and distributions in two adjoining regions of the South Atlantic Bight (SAB), Onslow Bay and nearshore Long Bay, were investigated over a 3-year period. Onslow Bay represents the northernmost region of the SAB, and receives very limited riverine influx. In contrast, Long Bay, just to the south, receives discharge from the Cape Fear River, draining the largest watershed within the State of North Carolina, USA. Northern Long Bay is a continental shelf ecosystem that has a nearshore area dominated by nutrient, turbidity and water-color loading from inputs from the river's plume. Average planktonic chlorophyll a concentrations ranged from 4.2 μg l −1 near the estuary mouth, to 3.1 μg l −1 7 km offshore in the plume's influence, to 1.9 μg l −1 at a non-plume station 7 km offshore to the northeast. Average areal planktonic chlorophyll a was approximately 3X that of benthic chlorophyll a at plume-influenced stations in Long Bay. In contrast, planktonic chlorophyll a concentrations in Onslow Bay were normally <0.50 μg l −1 at a nearshore (8 km) site, and <0.15 μg l −1 at sites located 45 and 100 km offshore. However, high water clarity ( K PAR 0.10–0.25 m −1) provides a favorable environment for benthic microalgae, which were abundant both nearshore (average 58.3 mg m −2) and to at least 45 km offshore in Onslow Bay (average 70.0 mg m −2) versus average concentrations of 10–12 mg m −2 for river-influenced areas of Long Bay. This provides evidence that much of the inner shelf food web in Onslow Bay is based on benthic microalgal production, in contrast to a plankton-based food web in northern Long Bay and more southerly areas of the SAB.

Benthic microalgae in coral reef sediments of the southern Great Barrier Reef, Australia

The abundance and productivity of benthic microalgae in coral reef sediments are poorly known compared with other, more conspicuous (e.g. coral zooxanthellae, macroalgae) primary producers of coral reef habitats. A survey of the distribution, biomass, and productivity of benthic microalgae on a platform reef flat and in a cross-shelf transect in the southern Great Barrier Reef indicated that benthic microalgae are ubiquitous, abundant (up to 995.0 mg chlorophyll (chl) a m−2), and productive (up to 110 mg O2 m−2 h−1) components of the reef ecosystem. Concentrations of benthic microalgae, expressed as chlorophyll a per surface area, were approximately 100-fold greater than the integrated water column concentrations of microalgae throughout the region. Benthic microalgal biomass was greater on the shallow water platform reef than in the deeper waters of the cross-shelf transect. In both areas the benthic microalgal communities had a similar composition, dominated by pennate diatoms, dinoflagellates, and cyanobacteria. Benthic microalgal populations were potentially nutrient-limited, based on responses to nitrogen and phosphorus enrichments in short-term (7-day) microcosm experiments. Benthic microalgal productivity, measured by O2 evolution, indicated productive communities responsive to light and nutrient availability. The benthic microalgal concentrations observed (92–995 mg chl a m−2) were high relative to other reports, particularly compared with temperate regions. This abundance of productive plants in both reef and shelf sediments in the southern Great Barrier Reef suggests that benthic microalgae are key components of coral reef ecosystems.

Transport pathways of microphytobenthos-originating organic carbon in the food web of an exposed hard bottom shore in the Seto Inland Sea, Japan

Benthic microalgal production has been clarified to be comparable to water column pro- duction in coastal shallow water ecosystems. The present study examined carbon transport pathways from microphytobenthos to predators in a food web of an exposed hard bottom shore in the Seto Inland Sea of Japan in winter, noting the intermediary role of suspension feeders in the carbon trans- port. Natural abundances of carbon and nitrogen stable isotopes were examined for epilithic organic matter (EOM), suspended particulate organic matter (SPOM), a macroalga, molluscs, crustaceans and fish. All the heterotrophs, including herbivores, suspension feeders and carnivores, showed high δ 13 C signatures of -15.5 to -9.7‰, relative to the δ 13 C of -20.7 to -19.7‰ for the SPOM collected from the offshore surface water as an indicator of phytoplankton. At the shoreline point, the SPOM showed high δ 13 C values of -16.0 to -13.0‰, overlapping with the values of -15.4 to -12.6‰ for the EOM indicative of microphytobenthos collected at the low sea level points, and abundantly included microalgae of the same species found in the epilithic matter. It was thus considered that resuspension of 13 C-enriched microphytobenthos increased the δ 13 C of SPOM in the surf zone and consequently increased the δ 13 C of the suspension feeders through their feeding on the suspended matter. The δ 15 N of carnivores, which overlapped in δ 13 C with these primary consumers, markedly increased with the trophic position. These results indicate that, not only grazing, but also suspension feeding functioned as transport pathways of the 13 C-enriched microphytobenthos in the food web.