Biogeochemical Processes In Sediments Research Articles

Impacts of Desalination Brine Discharge on Benthic Ecosystems.

Seawater reverse osmosis (SWRO) desalination facilities produce freshwater and, at the same time, discharge hypersaline brine that often includes various chemical additives such as antiscalants and coagulants. This dense brine can sink to the sea bottom and creep over the seabed, reaching up to 5 km from the discharge point. Previous reviews have discussed the effects of SWRO desalination brine on various marine ecosystems, yet little attention has been paid to the impacts on benthic habitats. This review comprehensibly discusses the effects of SWRO brine discharge on marine benthic fauna and flora. We review previous studies that indicated a suite of impacts by SWRO brine on benthic organisms, including bacteria, seagrasses, polychaetes, and corals. The effects within the discharge mixing zones range from impaired activities and morphological deformations to changes in the community composition. Recent modeling work demonstrated that brine could spread over the seabed, beyond the mixing zone, for up to several tens of kilometers and impair nutrient fluxes from the sediment to the water column. We also provide a possible perspective on brine's impact on the biogeochemical process within the mixing zone subsurface. Desalination brine can infiltrate into the sandy bottom around the discharge area due to gravity currents. Accumulation of brine and associated chemical additives, such as polyphosphonate-based antiscalants and ferric-based coagulants in the porewater, may change the redox zones and, hence, impact biogeochemical processes in sediments. With the demand for drinking water escalating worldwide, the volumes of brine discharge are predicted to triple during the current century. Future efforts should focus on the development and operation of viable technologies to minimize the volumes of brine discharged into marine environments, along with a change to environmentally friendly additives. However, the application of these technologies should be partly subsidized by governmental stakeholders to safeguard coastal ecosystems around desalination facilities.

Application of two-dimension, high-resolution evidences to reveal the biogeochemical process patterns of trace metals in reservoir sediments

Pollutions of trace metals (TMs) in reservoirs are blooming due to TMs were trapped efficiently in reservoir sediments by dams. Despite the mobilization of TMs in sediments have been well-documented, the patterns of biogeochemical processes occurred in sediments remain poorly understanding. Herein, a deep reservoir was selected to investigate the patterns of TMs biogeochemical processes in sediments by using high-resolution ZrO-Chelex-AgI diffusive gradient in thin films technique (HR-ZCA DGT) and the laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). 2-dimension high-resolution (2D-HR) images showed significant differential spatial enrichment of TMs (V, Mn, Fe, Co, Zn and Sb) in sediments, indicating strong heterogeneity in sediments. Correlations of TMs within microniches (diameter < 1 mm) in horizontal were usually different even contrast with that in vertical profile, suggesting distinct biogeochemical process patterns occurred in vertical vs. in horizontal. Further analyses from 2D-HR images showed the distributions of TMs in microniches reflected their mobilization that was driven by microenvironmental conditions. In contrast, distributions in sediment vertical profile recorded the diagenesis in different deposition depth. The diagenesis in sediment vertical is continuously accumulated by the discrete, microniches mobilization of TMs in horizontal. Collectively, our findings evidenced that 2D-HR data is an update complement to 1-dimension data for better interpret the biogeochemical process patterns of TMs in sediments, that have implication for water management to metals pollution in reservoir ecosystems.

Microbial Diversity and Authigenic Mineral Formation of Modern Bottom Sediments in the Littoral Zone of Lake Issyk-Kul, Kyrgyz Republic (Central Asia).

This article presents geochemical, mineralogical and microbiological characteristics of five samples of modern bottom sediments in the littoral zone of the high-mountain salty lake Issyk-Kul. The 16S rRNA gene sequencing method shows that the microbial community consists of organic carbon degraders (representatives of phyla: Proteobacteria, Chloroflexi, Bacteroidota and Verrucomicrobiota and families Anaerolineaceae and Hungateiclostridiaceae), photosynthetic microorganisms (representatives of Chloroflexi, phototrophic Acidobacteria, purple sulphur bacteria Chromatiaceae and cyanobacteria) and bacteria of the reducing branches of the sulphur biogeochemical cycle (representatives of Desulfobacterota, Desulfosarcinaceae and Desulfocapsaceae). The participation of microorganisms in processes in the formation of a number of authigenic minerals (calcite, framboidal pyrite, barite and amorphous Si) is established. The high diversity of microbial communities indicates the presence of labile organic components involved in modern biogeochemical processes in sediments. The active destruction of organic matter begins at the water-sediment interface.

Cable bacteria accelerate the anaerobic removal of pyrene in black odorous river sediments.

Cable bacteria play an essential role in biogeochemical processes in sediments by long-distance electron transport (LDET). A potential relationship has been found between cable bacteria and organic contaminant removal; however, the mechanisms remain unclear. In this study, the response of cable bacteria to pyrene was investigated in sediments with and without pyrene, and the effect of cable bacteria on pyrene removal was explored by connecting and blocking the paths of cable bacteria to the suboxic zones. The results showed that pyrene significantly influenced the microbial community structure and the composition of cable bacteria. The pyrene removal efficiencies significantly increased with the enrichment of cable bacteria, while sulfur-reducing microorganisms and aromatic compound degraders were also significantly enriched and correlated with cable bacteria abundance. Metagenomic analysis showed that cable bacteria have a potential LDET-bound acetate/formate respiratory pathway to gain energy. The presence of pyrene probably selects and enriches cable bacteria with a high tolerance to organic contaminants and changes the related functional microbial community, leading to the acceleration of pyrene removal. This study provides new insights into the interaction mechanisms between contaminants and cable bacteria, shedding light on the applications of cable bacteria in the bioremediation of contaminants in sediments.

Exploring the impacts of physicochemical characteristics and heavy metals fractions on bacterial communities in four rivers

Heavy metals contamination in sediment poses serious threats to bacterial communities that play critical roles in sediment biogeochemical processes. However, the physicochemical factors and the major heavy metals fractions that affect sediment bacterial communities are still unclear. Here, we performed heatmap and redundancy analyses to examine the effects of physico-chemical characteristics and heavy metals fractions on the sediment bacterial community from rivers in the UK (River Tyne and Ouseburn) and India (River Ganga and Yamuna). The results revealed that physicochemical characteristics and heavy metals fractions altered the diversity, richness, and structures of the bacterial community. Moreover, the fractions of Co, Zn, Pb, Cr, and Cu played significant roles in shaping the bacterial community structure, and physicochemical variables, particularly NH4+-N and NO2−-N, also influenced the bacterial diversity and structure. Firmicutes showed strong associations with both physicochemical factors and heavy metals fractions. Chloroflexi and Actinobacteriota can be used as biomarkers for Zn contamination. Overall, our study identified the significance of sediment chemical characteristics and heavy metals fractions in determining the bacterial community structure as well as bioremediation and environmental management of metals contaminated sites.

Specific characteristics of the microbial community in the groundwater fluctuation zone.

Groundwater level fluctuation is a common natural phenomenon that causes alternate changes in oxygen, moisture, and biogeochemical processes in sediments. Microbes are sensitive to these environmental changes. Therefore, a specific microbial community is proposed to form in the groundwater fluctuation zone (GFZ). The vertical distributions of microbial abundance, diversity, and functional microbes and genes in sediment profiles were investigated, focusing on the GFZ, using high-throughput 16S rRNA gene sequencing, qPCR, and the Functional Annotation of Prokaryotic Taxa (FAPROTAX) approach. The relationships between chemical variables and microbial community structure were investigated by redundancy analysis (RDA). Results showed that the microbial abundance and microbial community richness and diversity were higher in the sediments of the GFZ. The nitrate reducers prefer to stay just below the groundwater level in the GFZ. The predominant microbes in the GFZ functioned as nitrifiers and Fe-oxidizers. The specific community in the GFZ is mainly related to NO3- and Fe(III) in the sediment. Consequently, the biochemical processes nitrification and Fe- and Mn-oxidation sequentially happen above the nitrate-reduction zone near the groundwater level in the GFZ. These results provide new knowledge in the biogeochemistry cycle of the GFZ and its disturbance on the vertical distribution and transport of biogenic elements and contaminants.

Iron biogeochemistry in sediments on the western continental shelf of the Antarctic Peninsula

We examined iron biogeochemistry in continental margin sediments along the western side of the Antarctic Peninsula (WAP) to explore the connections between iron redox cycling and other sediment biogeochemical processes, and to specifically examine the role of sediments as a source of iron to support Southern Ocean primary productivity. Total iron (FeT) in WAP sediments ranged from ∼4 to 6 wt% Fe and showed no consistent depth variations. Total iron oxides (FeOx) represented ∼20–30% of FeT in WAP surface sediments and was comprised of two types of materials: Feox1, which is thought to contain amorphous, highly reactive, and poorly crystalline iron oxides such as ferrihydrite and lepidocrocite, and Feox2, which is thought to contain magnetite and less reactive, more crystalline iron oxides such as goethite and hematite. Absolute and relative concentrations of FeOx declined with sediment depth, presumably due to reductive dissolution, and there was also a major depth change in the speciation of FeOx. Near the sediment surface ∼30% of FeOx was Feox1, while below ∼30 cm Feox1 was almost completely consumed and therefore here virtually all FeOx was Feox2. Consistent with these observations, a simple reaction-transport model applied to the depth distributions of Feox1 and Feox2 iron in WAP shelf sediments suggested a ∼20-fold difference in the relative reactivity of these different oxides towards reductive dissolution (Feox1 > Feox2). At several stations profiles of pore water dissolved iron and solid phase Feox1 can be explained by steady state iron redox cycling. In contrast, profiles at other stations showed evidence of non steady-state iron redox cycling. Seasonal changes in the sediment organic carbon rain rate at these sites may explain this latter behavior. Differences in the behavior of iron at different sites may be related to the interplay between changes in sediment redox conditions and their impact on bioturbation and iron redox cycling. Sediment iron sources to the water column include dissolved iron (∼Fe2+) benthic fluxes as well as sediment resuspension processes that mix iron oxides (i.e., Feox1) found in surface sediments into the bottom waters. The impact these benthic iron sources will have on surface water iron concentrations (and eventually primary production) will depend on transport processes that move this iron from deeper waters to surface waters. They will also depend on chemical transformations in the water column that affect iron bioavailability. While sediment processes (in general) appear to be an important source of iron to Southern Ocean surface waters, there is large uncertainty in their magnitude. However, the results presented here demonstrate that the suspended sediment flux of amorphous iron oxides needs to be considered when discussing sediment iron sources to Southern Ocean surface waters.

Biogeochemical feedbacks to ocean acidification in a cohesive photosynthetic sediment

Ecosystem feedbacks in response to ocean acidification can amplify or diminish diel pH oscillations in productive coastal waters. Benthic microalgae generate such oscillations in sediment porewater and here we ask how CO2 enrichment (acidification) of the overlying seawater alters these in the absence and presence of biogenic calcite. We placed a 1-mm layer of ground oyster shells, mimicking the arrival of dead calcifying biota (+Calcite), or sand (Control) onto intact silt sediment cores, and then gradually increased the pCO2 in the seawater above half of +Calcite and Control cores from 472 to 1216 μatm (pH 8.0 to 7.6, CO2:HCO3− from 4.8 to 9.6 × 10−4). Porewater [O2] and [H+] microprofiles measured 16 d later showed that this enrichment had decreased the O2 penetration depth (O2-pd) in +Calcite and Control, indicating a metabolic response. In CO2-enriched seawater: (1) sediment biogeochemical processes respectively added and removed more H+ to and from the sediment porewater in darkness and light, than in ambient seawater increasing the amplitude of the diel porewater [H+] oscillations, and (2) in darkness, calcite dissolution in +Calcite sediment decreased the porewater [H+] below that in overlying seawater, reversing the sediment–seawater H+ flux and decreasing the amplitude of diel [H+] oscillations. This dissolution did not, however, counter the negative effect of CO2 enrichment on O2-pd. We now hypothesise that feedback to CO2 enrichment—an increase in the microbial reoxidation of reduced solutes with O2—decreased the sediment O2-pd and contributed to the enhanced porewater acidification.

Microbial Communities Involved in Methane, Sulfur, and Nitrogen Cycling in the Sediments of the Barents Sea.

A combination of physicochemical and radiotracer analysis, high-throughput sequencing of the 16S rRNA, and particulate methane monooxygenase subunit A (pmoA) genes was used to link a microbial community profile with methane, sulfur, and nitrogen cycling processes. The objects of study were surface sediments sampled at five stations in the northern part of the Barents Sea. The methane content in the upper layers (0–5 cm) ranged from 0.2 to 2.4 µM and increased with depth (16–19 cm) to 9.5 µM. The rate of methane oxidation in the oxic upper layers varied from 2 to 23 nmol CH4 L−1 day−1 and decreased to 0.3 nmol L−1 day−1 in the anoxic zone at a depth of 16–19 cm. Sulfate reduction rates were much higher, from 0.3 to 2.8 µmol L−1 day−1. In the surface sediments, ammonia-oxidizing Nitrosopumilaceae were abundant; the subsequent oxidation of nitrite to nitrate can be carried out by Nitrospira sp. Aerobic methane oxidation could be performed by uncultured deep-sea cluster 3 of gamma-proteobacterial methanotrophs. Undetectable low levels of methanogenesis were consistent with a near complete absence of methanogens. Anaerobic methane oxidation in the deeper sediments was likely performed by ANME-2a-2b and ANME-2c archaea in consortium with sulfate-reducing Desulfobacterota. Sulfide can be oxidized by nitrate-reducing Sulfurovum sp. Thus, the sulfur cycle was linked with the anaerobic oxidation of methane and the nitrogen cycle, which included the oxidation of ammonium to nitrate in the oxic zone and denitrification coupled to the oxidation of sulfide in the deeper sediments. Methane concentrations and rates of microbial biogeochemical processes in sediments in the northern part of the Barents Sea were noticeably higher than in oligotrophic areas of the Arctic Ocean, indicating that an increase in methane concentration significantly activates microbial processes.

Effects of dry-wet cycles on nitrous oxide emissions in freshwater sediments: a synthesis.

BackgroundSediments frequently exposed to dry-wet cycles are potential biogeochemical hotspots for greenhouse gas (GHG) emissions during dry, wet and transitional phases. While the effects of drying and rewetting on carbon fluxes have been studied extensively in terrestrial and aquatic systems, less is known about the effects of dry-wet cycles on N2O emissions from aquatic systems. As a notable part of lotic systems are temporary, and small lentic systems can substantially contribute to GHG emissions, dry-wet cycles in these ecosystems can play a major role on N2O emissions.MethodologyThis study compiles literature focusing on the effects of drying, rewetting, flooding, and water level fluctuations on N2O emissions and related biogeochemical processes in sediments of lentic and lotic ecosystems.ResultsN2O pulses were observed following sediment drying and rewetting events. Moreover, exposed sediments during dry phases can be active spots for N2O emissions. The general mechanisms behind N2O emissions during dry-wet cycles are comparable to those of soils and are mainly related to physical mechanisms and enhanced microbial processing in lotic and lentic systems. Physical processes driving N2O emissions are mainly regulated by water fluctuations in the sediment. The period of enhanced microbial activity is driven by increased nutrient availability. Higher processing rates and N2O fluxes have been mainly observed when nitrification and denitrification are coupled, under conditions largely determined by O2 availability.ConclusionsThe studies evidence the driving role of dry-wet cycles leading to temporarily high N2O emissions in sediments from a wide array of aquatic habitats. Peak fluxes appear to be of short duration, however, their relevance for global emission estimates as well as N2O emissions from dry inland waters has not been quantified. Future research should address the temporal development during drying-rewetting phases in more detail, capturing rapid flux changes at early stages, and further explore the functional impacts of the frequency and intensity of dry-wet cycles.

Sediment biogeochemistry along an oyster aquaculture chronosequence

Oyster aquaculture is expanding globally, and there has been a recent research surge examining how oyster farms alter coastal ecosystems. Yet, until now, the role of time in these studies has largely been missing. Here we used an in situ chronosequence approach to determine how the presence of oyster aquaculture (Crassostrea virginica) altered sediment nitrogen (N), oxygen (O2), and phosphorus (P) cycling. Overall we found that the sum of nitrogen fluxes increased significantly following addition of aquaculture, and switched from net N consumption (i.e. net nitrogen fixation: -14.41 µmol N m-2 h-1) to production (i.e. net denitrification: 553.57 µmol N m-2 h-1). Ammonium (NH4+) fluxes did not differ between bare sediment and oyster aquaculture. Additionally, both the magnitude of N2 and NH4+ fluxes oscillated on an annual scale of aquaculture age, but not predictably so. We observed significantly more variance (σ2) in dinitrogen and NH4+ fluxes in sediments beneath aquaculture, indicating increased non-linearity. O2 fluxes increased from Years 4 to 6, before returning to baseline conditions. There were no differences in sediment P cycling. This study demonstrates that sediment biogeochemical processes can become non-linear under the pressure of oyster aquaculture, and this non-linearity likely has important implications for ecosystem function.

Development of a new sediment flux model – Application in Chesapeake Bay

Sediment biogeochemical processes impact pelagic ecosystems when the sediment receives particulate organic matter (POM) deposited from the water column above, consumes dissolved oxygen, and returns inorganic nutrients to the bottom water. However, an easy-to-apply sediment model is still lacking for pelagic biogeochemical modeling. In this study, we proposed a mass conservative, process-based and easy-to-apply sediment flux model (SFM) that endeavors to simplify Di Toro (2001)’s sediment flux model (DSFM) while maintaining the ability to model sediment fluxes in multiple environments (e.g., anoxic, normoxic, variable salinity, high and low organic matter deposition, etc.). It adopts a vertically integrated approach, and has a relatively simple structure with limited number of parameters. The mineralization scheme for POM is based on bacterial growth, which can resolve the diagenesis processes on different timescales. The inhibition effects of oxygen on denitrification of NO3 and desorption of PO4 are modeled using exponential functions. This new SFM is then implemented in the Chesapeake Bay to simulate sediment oxygen demand (SOD) and sediment NH4/NO3/PO4/Silica fluxes across the sediment–water interface. To estimate POM depositional fluxes from the water column, a new method is introduced that links surficial sediment and bottom-water POM concentrations. Further integration of this method into the SFM turns the model calibration into a mass conservative and self-consistent process, which is well constrained by observations. The modeled sediment fluxes from the new SFM match observations across seasons and stations with correlation coefficient ranging from 0.3 to 0.7. The successful application of SFM in Chesapeake Bay and subsequent analysis show that the new SFM has many advantages including mass conservation, high temporal resolution, relatively simple structure, and easy implementation. The SFM is a good tool for linking pelagic ecosystem models to sediment biogeochemical processes.

Ocean acidification and hypoxia alter organic carbon fluxes in marine soft sediments.

Anthropogenic stressors can alter the structure and functioning of infaunal communities, which are key drivers of the carbon cycle in marine soft sediments. Nonetheless, the compounded effects of anthropogenic stressors on carbon fluxes in soft benthic systems remain largely unknown. Here, we investigated the cumulative effects of ocean acidification (OA) and hypoxia on the organic carbon fate in marine sediments, through a mesocosm experiment. Isotopically labelled macroalgal detritus (13 C) was used as a tracer to assess carbon incorporation in faunal tissue and in sediments under different experimental conditions. In addition, labelled macroalgae (13 C), previously exposed to elevated CO2 , were also used to assess the organic carbon uptake by fauna and sediments, when both sources and consumers were exposed to elevated CO2 . At elevated CO2 , infauna increased the uptake of carbon, likely as compensatory response to the higher energetic costs faced under adverse environmental conditions. By contrast, there was no increase in carbon uptake by fauna exposed to both stressors in combination, indicating that even a short-term hypoxic event may weaken the ability of marine invertebrates to withstand elevated CO2 conditions. In addition, both hypoxia and elevated CO2 increased organic carbon burial in the sediment, potentially affecting sediment biogeochemical processes. Since hypoxia and OA are predicted to increase in the face of climate change, our results suggest that local reduction of hypoxic events may mitigate the impacts of global climate change on marine soft-sediment systems.

Modelling biogeochemical processes in sediments from the north-western Adriatic Sea: response to enhanced particulate organic carbon fluxes

Abstract. This work presents the result of a study carried out in the north-western Adriatic Sea, by combining two different types of biogeochemical models with field sampling efforts. A longline mussel farm was taken as a local source of perturbation to the natural particulate organic carbon (POC) downward flux. This flux was first quantified by means of a pelagic model of POC deposition coupled to sediment trap data, and its effects on sediment bioirrigation capacity and organic matter (OM) degradation pathways were investigated constraining an early diagenesis model by using original data collected in sediment porewater. The measurements were performed at stations located inside and outside the area affected by mussel farm deposition. Model-predicted POC fluxes showed marked spatial and temporal variability, which was mostly associated with the dynamics of the farming cycle. Sediment trap data at the two sampled stations (inside and outside of the mussel farm) showed average POC background flux of 20.0–24.2 mmol C m−2 d−1. The difference of organic carbon (OC) fluxes between the two stations was in agreement with model results, ranging between 3.3 and 14.2 mmol C m−2 d−1, and was primarily associated with mussel physiological conditions. Although restricted, these changes in POC fluxes induced visible effects on sediment biogeochemistry. Observed oxygen microprofiles presented a 50 % decrease in oxygen penetration depth (from 2.3 to 1.4 mm), accompanied by an increase in the O2 influx at the station below the mussel farm (19–31 versus 10–12 mmol O2 m−2 d−1) characterised by higher POC flux. Dissolved inorganic carbon (DIC) and NH4+ concentrations showed similar behaviour, with a more evident effect of bioirrigation underneath the farm. This was confirmed through constraining the early diagenesis model, of which calibration leads to an estimation of enhanced and shallower bioirrigation underneath the farm: bioirrigation rates of 40 yr−1 and irrigation depth of 15 cm were estimated inside the shellfish deposition footprint versus 20 yr−1 and 20 cm outside. These findings were confirmed by independent data on macrofauna composition collected at the study site. Early diagenesis model results indicated a larger organic matter mineralisation below the mussel farm (11.1 versus 18.7 mmol m−2 d−1), characterised by similar proportions between oxic and anoxic degradation rates at the two stations, with an increase in the absolute values of oxygen consumed by OM degradation and reduced substances re-oxidation underneath the mussel farm.

Photo-induced toxicity in early life stage fiddler crab (Uca longisignalis) following exposure to Deepwater Horizon oil.

The 2010 explosion of the Deepwater Horizon (DWH) oil rig led to the release of millions of barrels of oil in the Gulf of Mexico. Oil in aquatic ecosystems exerts toxicity through multiple mechanisms, including photo-induced toxicity following co-exposure with UV radiation. The timing and location of the spill coincided with both fiddler crab reproduction and peak yearly UV intensities, putting early life stage fiddler crabs at risk of injury due to photo-induced toxicity. The present study assessed sensitivity of fiddler crab larvae to photo-induced toxicity during co-exposure to a range of environmentally relevant dilutions of high-energy water accommodated fractions of DWH oil, and either <10, 50, or 100% ambient sunlight, achieved with filters that allowed for variable UV penetration. Solar exposures (duration: 7-h per day) were conducted for two consecutive days, with a dark recovery period (duration: 17-h) in between. Survival was significantly decreased in treatments the presence of >10% UV and relatively low concentrations of oil. Results of the present study indicate fiddler crab larvae are sensitive to photo-induced toxicity in the presence of DWH oil. These results are of concern, as fiddler crabs play an important role as ecosystem engineers, modulating sediment biogeochemical processes via burrowing action. Furthermore, they occupy an important place in the food web in the Gulf of Mexico.

Links between contaminant hotspots in low flow estuarine systems and altered sediment biogeochemical processes

The urbanisation of coastal zones is a major threat to the health of global estuaries and has been linked to increased contamination (e.g. metals) and excess organic matter. Urban stormwater networks collect and funnel contaminants into waterways at point sources (e.g. stormdrains). Under dry, low flow conditions, these stormwater contaminants can accumulate in sediments over time and result in modifications to benthic sediment biogeochemical processes. To quantify these processes, this field study measured differences in benthic metabolism (CR, GPP, NEM) and sediment-water nutrient fluxes (NH3, NOx, PO4) associated with stormdrains (0 m, 200 m and 1000 m away) and increased water-retention (embayments vs channels). Significant changes to benthic metabolism were detected with distance from stormdrains, and with differences in water-retention rates, above natural spatial and temporal variation. Oxygen consumption was ∼50% higher at stormdrains (0 m) compared to 1000 m away and >70% higher at stormdrains (0 m) located in embayments compared to channels. Oxygen production also appeared to decrease with distance from stormdrains in embayments, but patterns were variable. These changes to benthic metabolism were of a magnitude expected to influence benthic nutrient cycling, but NH3, NOx and PO4 fluxes were generally low, and highly spatially and temporally variable. Overall, metal (Cu) contamination explained most of the variation in sediment biogeochemical processes between embayments and channels, while sediment grain size explained differences in fluxes with distance from stormdrains. Importantly, although there was evidence of increased productivity associated with stormdrains, we also detected evidence of early hypoxia suggesting that systems with legacy stormwater contaminants exist on a tipping point. Future work should investigate changes to sediment processes after a major rainfall event, when large and sudden inputs of potentially toxic contaminants occur. Monitoring benthic O2 fluxes could be a sensitive measure of ecological change under these conditions.

A biophysical representation of seagrass growth for application in a complex shallow-water biogeochemical model

Seagrasses are a critical component of the healthy functioning of many coastal marine ecosystems. Capturing the dynamics of seagrass communities requires both a detailed representation of processes such as seagrass nutrient uptake and photosynthesis, as well as models of light penetration, water column and sediment biogeochemical processes and other ecosystem characteristics that determine the environmental state. Here we develop a new two-state, 13-parameter seagrass model with the aim of providing sufficient detail to represent light and nutrient limitation, but simple enough to be coupled into a 60 state variable biogeochemical model. The novel formulation is built around a nitrogen-specific leaf area parameter, Ω, that is well-constrained and is used in calculating both the rate of photosynthesis and the fraction of the seafloor covered by seagrass, Aeff, where Aeff=1−exp(−ΩSGA) and SGA is the aboveground areal seagrass biomass. The model also contains terms for the uptake of nutrients from multiple layers of varying-porosity sediments, translocation of organic matter between leaves and roots, respiration and simple mortality terms. The model is applied to Gladstone Harbour, a macro-tidal sub-tropical estuary in northeast Australia, and is able to simulate realistic spatial seagrass distributions. A simplified form of the model is derived, which can be used to predict seagrass light-limited growth based on five measurable species-specific parameters (maximum growth rate, mortality rate, compensation irradiance, leaf blade angle and nitrogen-specific leaf area). The steady-state percent coverage of seagrass achieved at varying light levels and mortality intensity is calculated as a means of understanding the dynamics of the new seagrass model.

Seagrass derived organic matter influences biogeochemistry, microbial communities, and seedling biomass partitioning in seagrass sediments

Seedling establishment is a crucial life history stage in seagrasses, yet factors that affect seedling health are poorly characterized. We investigated if organic matter (OM) additions to sediments provided nutritional benefits for seagrass seedlings through microbial degradation. We tested the effects of sedimentary OM additions on Posidonia australis seedlings growing in tank cultures. We focussed on sediment biogeochemical processes and microbial communities that may impact seedling growth and physiology. Enrichment of sediments with OM changed microbial community composition (DNA-ARISA) and a significant increase in hydrolytic enzyme expression. Total seedling biomass did not differ between OM treatments, but above:belowground biomass increased with OM enrichment. Nitrogen and phosphorus concentration of seagrass leaves was lower with increasing OM. Seagrass derived OM has been considered a refractory store of carbon, yet here we show its deposition into sediments significantly alters belowground conditions. Remineralization of the OM changes both physical and chemical nature of sediments that leads to greater biochemical activity, change in microbial communities and greater investment into above ground photosynthetic biomass. The presence of OM may assist seagrass seedling survival during early development by enhancing root branching and stability in sediments, but is unlikely to provide nutritional benefits.

Benthic community response to sediment organic enrichment by Mediterranean fish farms: Case studies

There is a great deal of scientific information about the biogeochemical processes in sediments near fish farms and how those processes may be driven under nutrient enrichment to environmental negative effects, as biodiversity decrease is observed in benthic assemblages. Within an adaptive management framework, a good monitoring program can be used to adjust farm management to avert serious and persistent impacts to the sediment. Macrobenthic community represents a good indicator able to detect environmental effects resulting from organic enrichment and, based on this, was selected to monitor Italian marine fish farms. In order to assess the environmental sediment disturbance resulting from marine aquaculture along the Italian coasts three marine fish farms were considered based on their different environmental features: two located in the Tyrrhenian Sea and one in the Adriatic Sea. The selected cage fish farms produce European sea bass Dicentrarchus labrax (L.), gilthead sea-bream Sparus aurata (L.) and meagre Argyrosomus regius (A.) The study ranges from 2001 to 2012, at different times in each fish farm, and was planned according to the fattening cycles. The samples for benthic community analyses were collected by means of a van Veen grab in selected stations located from 0 to 1000m from the sea cages, the farthest as control stations. In total 134 stations were sampled for more of 20,000 individuals belonging to more than 450 species. Data analyses were performed with statistical methods and biotic indices were calculated. The results showed changes in benthic communities at the stations located just beneath the cages and connected with high cultured biomass. The impacted sites showed loss of biodiversity and increase of opportunistic species such as Capitella Sp. I and Nebalia strausi. Changes in the benthic communities were more or less evident in relation to substrata composition, hydrodynamic conditions and cultured biomass. The observed impact was limited to a few tens of meters around the sea cages of all the three studied fish farms.

More than propagule pressure: Successful invading algae have physiological adaptations suitable to anthropogenically changing nutrient environments

While increased propagule pressure may increase the possibility of success of an invading species, success is also a function of the environment to which the organisms have been introduced and the physiological strategies of the invading organisms and how those strategies relate to those of the native fauna or flora. Using examples of altered nutrients in aquatic environments, the effects of environment on the success of harmful algae are herein considered. Many harmful algae are introduced species, especially via ballast water. While increasing numbers and frequency of harmful algal blooms in eutrophic waters may be comparatively easy to understand—more nutrients, more biomass, more opportunities for species to proliferate—one of the more perplexing issues is why some harmful algal bloom species seem to become more prevalent under conditions of disproportionate nutrient supply. Focusing here specifically on the case of excess nitrogen loading relative to that of phosphorus, there are a number of key adaptive strategies that may allow certain types of harmful algal blooms to thrive. Successful invaders may be smaller in cell size, thereby reducing phosphorus cellular demands, and they may have nutrient substitution or acquisition strategies allowing them to use a source of nutrient not otherwise available to competitors. They may also create local environmental changes, such as increases in pH that detrimentally affect competitors while altering sediment biogeochemical processes increasing efflux of the otherwise limiting nutrient. Finally, their growth and cellular elemental stoichiometry also affect food quality for grazers, including toxin production, which has been shown in numerous species to increase when the cells are not grown under nutrient-balanced conditions. Through their detrimental effects on grazers, they may further destabilize ecosystem environments increasing susceptibility for additional ecosystem changes, including creating environmental conditions suitable for additional invasions to occur.