Resuspension by fish facilitates the transport and redistribution of coastal sediments

Author Posting. © Association for the Sciences of Limnology and Oceanography, 2012. This article is posted here by permission of Association for the Sciences of Limnology and Oceanography for personal use, not for redistribution. The definitive version was published in Limnology and Oceanography 57 (2012): 945-958, doi:10.4319/lo.2012.57.4.0945.Oxygen availability restricts groundfish to the oxygenated, shallow margins of Saanich Inlet, an intermittently anoxic fjord in British Columbia, Canada. New and previously reported 210Pb measurements in sediment cores compared with flux data from sediment traps indicate major focusing of sediments from the oxygenated margins to the anoxic basin seafloor. We present environmental and experimental evidence that groundfish activity in the margins is the major contributor to this focusing. Fine particles resuspended by groundfish are advected offshore by weak bottom currents, eventually settling in the anoxic basin. Transmittance and sediment trap data from the water column show that this transport process maintains an intermediate nepheloid layer (INL) in the center of the Inlet. This INL is located above the redox interface and is unrelated to water density shifts in the water column. We propose that this INL is shaped by the distribution of groundfish (as resuspension sources) along the slope and hence by oxygen availability to these fish. We support this conclusion with a conceptual model of the resuspension and offshore transport of sediment. This fish-induced transport mechanism for sediments is likely to enhance organic matter decomposition in oxygenated sediments and its sequestration in anoxic seafloors.The VENUS Project and University of Victoria supported the ship and submersible time for field experiments, and the U.S. Geological Survey and Coastal and Marine Geological Program generously supported J.C. The project was supported by Discovery Grants from the Natural Sciences and Engineering Research Council of Canada to V.T. and P.S. and a Yohay Ben-Nun fellowship and Moshe Shilo Center for Marine Biogeochemistry Fund award to T.K

Groundfish overfishing, diatom decline, and the marine silica cycle : lessons from Saanich Inlet, Canada, and the Baltic Sea cod crash

Author Posting. © American Geophysical Union, 2009. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 23 (2009): GB4032, doi:10.1029/2008GB003416.In this study, we link groundfish activity to the marine silica cycle and suggest that the drastic mid-1980s crash of the Baltic Sea cod (Gadus morhua) population triggered a cascade of events leading to decrease in dissolved silica (DSi) and diatom abundance in the water. We suggest that this seemingly unrelated sequence of events was caused by a marked decline in sediment resuspension associated with reduced groundfish activity resulting from the cod crash. In a study in Saanich Inlet, British Columbia, Canada, we discovered that, by resuspending bottom sediments, groundfish triple DSi fluxes from the sediments and reduce silica accumulation therein. Using these findings and the available oceanographic and environmental data from the Baltic Sea, we estimate that overfishing and recruitment failure of Baltic cod reduced by 20% the DSi supply from bottom sediments to the surface water leading to a decline in the diatom population in the Baltic Sea. The major importance of the marginal ocean in the marine silica cycle and the associated high population density of groundfish suggest that groundfish play a major role in the silica cycle. We postulate that dwindling groundfish populations caused by anthropogenic perturbations, e.g., overfishing and bottom water anoxia, may cause shifts in marine phytoplankton communities.We acknowledge the VENUS Project, University of Victoria, for supporting the ship and submersible time for field experiments and USGS, CMGP, for support to J.C. Additional funding from NSERC Canada and from the Canada Research Chairs Foundation to V.T.; a Rothschild fellowship to G.Y.; and a Yohay Ben-Nun fellowship and Moshe Shilo Center for Marine Biogeochemistry fund to T.K. are also acknowledged

Sandbar Breaches Control of the Biogeochemistry of a Micro-Estuary

Micro-estuaries in semi-arid areas, despite their small size (shallow depth of a few meters, length of a few kilometers, and a surface area of less than 1 km2) are important providers of ecosystem services. Despite their high abundance, tendency to suffer from eutrophication and vulnerability to other anthropogenic impacts, such systems are among the least studied water bodies in the world. In low tidal amplitude regions, micro-estuaries often have limited rate of sea-river water exchange, somewhat similar to fjord circulation, caused by a shallow sandbar forming at the coastline. The long-term study, we report here was inspired by the idea that, due to their small size and low discharges regime, relatively small interventions can have large effects on micro-estuaries. We used a stationary array of sensors and detailed monthly water sampling to characterize the Alexander estuary, a typical micro-estuary in the S.E. Mediterranean, and to identify the main stress factors in this aquatic ecosystem. The Alexander micro-estuary is stratified throughout the year with median bottom salinity of 18 PSU. Prolonged periods of hypoxia were identified as the main stress factor. Those were alleviated by breaching of the sandbar at the estuary mouth by sea-waves or stormwater runoff events (mostly during winter) that flush the anoxic bottom water. Analysis of naturally occurring sandbar breaches, and an artificial breach experiment indicate that the current oxygen consumption rate of the Alexander micro-estuary is too high to consider sandbar breaches as a remedy for the anoxia. Nevertheless, it demonstrates and provides the tools to assess the feasibility of small-scale interventions to control micro-estuaries hydrology and biogeochemistry

In situ Pumping Rate of 20 Marine Demosponges Is a Function of Osculum Area

Sponges play a key role in the transfer of energy and nutrients into many benthic ecosystems, and the volume of water they process is an important regulator of these fluxes. Theoretical scaling relationships between sponge volume, osculum cross-sectional area, and pumping rates were recently proposed and confirmed for small sponge specimens in the lab. To examine how these relationships apply to field populations we measured, in situ, the pumping rate (PR) of 20 species representative of different morphologies and host types (high- and low-microbial-abundance, HMA and LMA) from temperate and tropical regions. The total oscula area (∑OSA) increased allometrically with sponge volume (V) exhibiting similar exponents (∑OSA=aVb, b ranging 0.6–0.7) for all species, except for tropical HMAs (b = 0.99). Osculum flow rate (OFR) also increased allometrically with OSA and oscula of the same size pumped at the same rate irrespective of sponge volume. As a result, and in contrast to former reports, the PR of most of the sponges increased allometrically (PR=a∑OSAb) with scaling exponent b≈0.75, whereas PR of tropical HMAs increased isometrically. Osculum jet speed declined with the increase in the OSA for most species. The number of oscula and their OSA were the best predictors of the PR in sponges, explaining 75–94% of the in situ variation in PR throughout the natural range of sponge size. The pumping rate of a sponge population can be estimated by measuring the osculum density and cross-sectional area distribution once the relationships between the OSA and OFR are established for each species

Intermittent hypoxia and prolonged suboxia measured in situ in a marine sponge

High Microbial Abundance (HMA) sponges constitute a guild of suspension-feeding sponges that host vast populations of symbiotic microbes. These symbionts mediate a complex series of biogeochemical transformations that fuel the holobiont’s metabolism. Although sponges are aerobic animals, suboxic and anaerobic bacteria are known to reside within their bodies. However, little is known about the chemical characteristics of the sponge environment in which they occur and almost no data are available regarding the dissolved oxygen (DO) dynamics inside the holobiont in its natural habitat. In this study we examined the oxygen dynamics in situ in the HMA sponge Theonella swinhoei. A submersed data-logging system equipped with micro-sensors was used to continuously record DO concentrations inside the sponge body and in its outflowing water for up to 48 hours. Actively pumping sponges exhibited high DO removal rates punctuated with short bursts of extreme DO uptake (>90 µmol DO Lpumped-1), never before observed in sponges. Such a high DO removal rate indicates the consumption of a considerable amount of reduced matter, far exceeding the available sources in the surrounding water of the oligotrophic coral-reef ecosystem inhabited by this sponge. The inner body of the sponge remained suboxic throughout the experiments, with short events of further rapid DO concentration decline. Moreover, DO concentrations measured in the body and in the outflowing water were found to be uncorrelated. Our findings support a previous hypothesis of bacterial symbiont farming by the sponge as a potential source for acquiring reduced material. Moreover, this suggests a complex and highly localized control of the holobiont’s metabolism, probably associated with the microbial community’s metabolism. Our results indicate that temporal micro-environments exist in the sponge at alternating locations, providing suitable conditions for the activity of its anaerobic microbial symbionts

Data from: Glass sponge reefs as a silicon sink

Glass sponge reefs concentrate large amounts of biological silicon (Si) over relatively small areas of the seafloor. We examined the role of glass sponges in biological silicon (Si) cycling by calculating a Si budget for 3 glass sponge reefs (Howe, Fraser, and Galiano) in the Strait of Georgia (SOG), British Columbia, Canada. The main reef-forming glass sponge Aphrocallistes vastus is heavily silicified, with 80% of its dry weight composed of biogenic silica (bSi). We used a combination of field sampling and surveys with a remote-operated vehicle to estimate the volume, mass, and bSi content of the reefs. BSi content ranged from 7 to 11 kg m−2 among reefs, amounting to a total of 915 t of bSi locked in the exposed portion of the 3 reefs. Water column measurements of dissolved Si (dSi) indicated that the SOG is a region of high dSi, with average dSi concentrations of 50 μmol l−1 in waters over the reefs. The skeletons of glass sponges showed very little dissolution after 8 mo immersion in seawater, as determined by changes in dSi in samples and scanning electron microscopy of the spicules. In contrast, diatom frustules, the main source of bSi in surface waters of the SOG, were ~200 times more soluble. Our calculations of Si flux suggest that glass sponge reefs can equate to 65% of the dSi reservoir (3.6 × 109 mol Si) in the SOG and represent a substantial Si sink in the continental shelf waters of the northeastern Pacific Ocean

Data from: Glass sponge reefs as a silicon sink

Glass sponge reefs concentrate large amounts of biological silicon (Si) over relatively small areas of the seafloor. We examined the role of glass sponges in biological silicon (Si) cycling by calculating a Si budget for 3 glass sponge reefs (Howe, Fraser, and Galiano) in the Strait of Georgia (SOG), British Columbia, Canada. The main reef-forming glass sponge Aphrocallistes vastus is heavily silicified, with 80% of its dry weight composed of biogenic silica (bSi). We used a combination of field sampling and surveys with a remote-operated vehicle to estimate the volume, mass, and bSi content of the reefs. BSi content ranged from 7 to 11 kg m−2 among reefs, amounting to a total of 915 t of bSi locked in the exposed portion of the 3 reefs. Water column measurements of dissolved Si (dSi) indicated that the SOG is a region of high dSi, with average dSi concentrations of 50 μmol l−1 in waters over the reefs. The skeletons of glass sponges showed very little dissolution after 8 mo immersion in seawater, as determined by changes in dSi in samples and scanning electron microscopy of the spicules. In contrast, diatom frustules, the main source of bSi in surface waters of the SOG, were ~200 times more soluble. Our calculations of Si flux suggest that glass sponge reefs can equate to 65% of the dSi reservoir (3.6 × 109 mol Si) in the SOG and represent a substantial Si sink in the continental shelf waters of the northeastern Pacific Ocean

Prey taxonomy rather than size determines salp diets

International audienceSalps are gelatinous planktonic suspension feeders that filter large volumes of water in the food-dilute open ocean. Their life cycle allows periodic exponential growth and population blooms. Dense swarms of salps have a high grazing impact that can deplete the photic zone of phytoplankton and export huge quantities of organic matter to the deep sea. Previous studies described their feeding manner as mostly nonselective, with larger particles retained at higher efficiencies than small particles. To examine salp diets, we used direct in situ sampling (InEx method) of undisturbed solitary Salpa maxima. Aggregates ("chains") of Salpa fusiformis and Thalia democratica were studied using in situ incubations. Our findings suggest that in situ feeding rates are higher than previously reported and that cell removal is size independent with $ 1 μm picoeukaryotes preferentially removed over both larger eukaryotes and smaller bacteria. The prey : predator size ratios we measured (1 : 10 4-1 : 10 5) are an order of magnitude smaller than previously reported values and to the best of our knowledge, are the smallest values reported so far for any planktonic suspension feeders. Despite differences among the three species studied, they had similar prey preferences with no correlation between salp body length and prey size. Our findings shed new light on prey : predator relationships in planktonic systems-in particular, that factors other than size influence filtration efficiency-and suggest that in situ techniques should be devised and applied for the study of suspension feeding in the ocean

Not‑so‑simple sieving by ascidians : Re‑examining particle capture at the mesh and organismal scales

The particle capture mechanisms of biological filters determine the particle spectrum that is ingested by filter-feeding animals. Although ascidian feeding has been extensively investigated, the organismal-scale fluid dynamics and mesh-scale particle-filter interactions are not fully characterized. Fluorescein dye visualization of flow through the branchial sac of the ascidian Ciona intestinalis showed organismal-scale flow was laminar and moved both parallel and perpendicular to the mucous mesh. Endoscopic investigations of Herdmania momus revealed the mesh-scale filtration process, including the pre-capture velocities, particle approach angles, and mesh behavior. The mesh speed was variable (range 0–0.4 mm s−1). To determine how particle shape affects hydrosol capture, Styela plicata was fed differently shaped polystyrene particles (ellipsoids and spheres); sampling the inhaled and exhaled water revealed that microellipsoids (0.3 × 0.7 μm) were captured at significantly lower efficiency (32%) than 1 μm microspheres (86%). The capture efficiency of microellipsoids resembled that of microspheres with a diameter similar to the microellipsoids’ minor axis (0.3 μm, 31%) suggesting that the minimum diameter of ellipsoidal particles determines the capture efficiency. Flow near the filter was parallel to the mesh even ~ 10 s of micrometers away, implicating a “crossflow” component to ascidian filtration, where the fluid being filtered is directed along the surface of the filter rather than exclusively perpendicular to it. Collectively, these results suggest that ascidian filtration acts as a hybrid-flow filtration system rather than a classical direct sieve