D. B. Sivyer

Nutrient fluxes through the Humber estuary - past, present and future

Abstract The geomorphology of the present day and Holocene (3000 years ago) Humber estuary, United Kingdom, are described. More than 90% of the intertidal area and sediment accumulation capacity of the estuary has been lost to reclamation over this period. A similar situation prevails in many other urbanized estuaries. Nutrient budgets for the modern estuary are presented demonstrating little trapping of nutrients, due to the loss of intertidal areas. A speculative budget for the Humber during the Holocene is constructed, which suggests that the estuary was then an efficient sink for nitrogen and phosphorus. A budget is presented describing how nutrient cycling might operate in the Humber with contemporary nutrient loadings, but with the pre-reclamation geography. This suggests that in this form the estuary would significantly attenuate nutrient fluxes to the North Sea. The results are discussed in terms of options for managed realignment of estuaries in response to predicted sea-level rise.

Modelling potential impacts of bottom trawl fisheries on soft sediment biogeochemistry in the North Sea.

Bottom trawling causes physical disturbance to sediments particularly in shelf areas. The disturbance due to trawling is most significant in deeper areas with softer sediments where levels of natural disturbance due to wave and tidal action are low. In heavily fished areas, trawls may impact the same area of seabed more than four times per year. A single pass of a beam trawl, the heaviest gear routinely used in shelf sea fisheries, can kill 5–65% of the resident fauna and mix the top few cm of sediment. We expect that sediment community function, carbon mineralisation and biogeochemical fluxes will be strongly affected by trawling activity because the physical effects of trawling are equivalent to those of an extreme bioturbator, and yet, unlike bioturbating macrofauna, trawling does not directly contribute to community metabolism. We used an existing box-model of a generalised soft sediment system to examine the effects of trawling disturbance on carbon mineralisation and chemical concentrations. We contrasted the effects of a natural scenario, where bioturbation is a function of macrobenthos biomass, with an anthropogenic impact scenario where physical disturbance results from trawling rather than the action of bioturbating macrofauna. Simulation results suggest that the effects of low levels of trawling disturbance will be similar to those of natural bioturbators but that high levels of trawling disturbance prevent the modelled system from reaching equilibrium due to large carbon fluxes between oxic and anoxic carbon compartments. The presence of macrobenthos in the natural disturbance scenario allowed sediment chemical storage and fluxes to reach equilibrium. This is because the macrobenthos are important carbon consumers in the system whose presence reduces the magnitude of available carbon fluxes. In soft sediment systems, where the level physical disturbance due to waves and tides is low, model results suggest that intensive trawling disturbance could cause large fluctuations in benthic chemical fluxes and storage.

Smartbuoy: A marine environmental monitoring buoy with a difference

Abstract High frequency measurements of physical, chemical and biological variables are being routinely made employing a moored platform (SmartBuoy) in UK shelf-seas. The rationale for the design and configuration of SmartBuoy are described together with details of the system control, software environment and telemetry capability. An 18- month time series of results, showing different scales of variability from annual, through seasonal to daily of a range of measured variables, is presented. Future developments to extend the capability of the current version of SmartBuoy to measure wave direction and current speed are described together with a description of the next generation SmartBuoy 3 platform.

New approaches to improve the detection of eutrophication in UK coastal waters

Robust assessments of eutrophication are necessary to meet the requirements of a range of international (OSPAR) and EU legislative drivers. To meet these needs EU states have developed marine monitoring programmes designed to allow the application of specified assessment procedures. The UK has reviewed its approach to monitoring eutrophication and has identified a range of future requirements to ensure the evidence base for assessment is robust and the underpinning science is in place. This paper describes the pilot application of in situ monitoring technology (SmartBuoy). Currently, two buoys are deployed in the southern North Sea and a third in Liverpool Bay (Irish Sea). The network of SmartBuoys returns data on physical, chemical and biological variables in near real-time (www.cefas.co.uk/monitoring). The rationale for system and network design will be described. Data from the multi-year time series will be presented and their subsequent use in assessments of eutrophication will be described.

An approach for the identification of exemplar sites for scaling up targeted field observations of benthic biogeochemistry in heterogeneous environments

Continental shelf sediments are globally important for biogeochemical activity. Quantification of shelf-scale stocks and fluxes of carbon and nutrients requires the extrapolation of observations made at limited points in space and time. The procedure for selecting exemplar sites to form the basis of this up-scaling is discussed in relation to a UK-funded research programme investigating biogeochemistry in shelf seas. A three-step selection process is proposed in which (1) a target area representative of UK shelf sediment heterogeneity is selected, (2) the target area is assessed for spatial heterogeneity in sediment and habitat type, bed and water column structure and hydrodynamic forcing, and (3) study sites are selected within this target area encompassing the range of spatial heterogeneity required to address key scientific questions regarding shelf scale biogeochemistry, and minimise confounding variables. This led to the selection of four sites within the Celtic Sea that are significantly different in terms of their sediment, bed structure, and macrofaunal, meiofaunal and microbial community structures and diversity, but have minimal variations in water depth, tidal and wave magnitudes and directions, temperature and salinity. They form the basis of a research cruise programme of observation, sampling and experimentation encompassing the spring bloom cycle. Typical variation in key biogeochemical, sediment, biological and hydrodynamic parameters over a pre to post bloom period are presented, with a discussion of anthropogenic influences in the region. This methodology ensures the best likelihood of site-specific work being useful for up-scaling activities, increasing our understanding of benthic biogeochemistry at the UK-shelf scale.

SCUBA divers as oceanographic samplers: The potential of dive computers to augment aquatic temperature monitoring

Monitoring temperature of aquatic waters is of great importance, with modelled, satellite and in-situ data providing invaluable insights into long-term environmental change. However, there is often a lack of depth-resolved temperature measurements. Recreational dive computers routinely record temperature and depth, so could provide an alternate and highly novel source of oceanographic information to fill this data gap. In this study, a citizen science approach was used to obtain over 7,000 scuba diver temperature profiles. The accuracy, offset and lag of temperature records was assessed by comparing dive computers with scientific conductivity-temperature-depth instruments and existing surface temperature data. Our results show that, with processing, dive computers can provide a useful and novel tool with which to augment existing monitoring systems all over the globe, but especially in under-sampled or highly changeable coastal environments.

Oxygen dynamics in shelf seas sediments incorporating seasonal variability

Shelf sediments play a vital role in global biogeochemical cycling and are particularly important areas of oxygen consumption and carbon mineralisation. Total benthic oxygen uptake, the sum of diffusive and faunal mediated uptake, is a robust proxy to quantify carbon mineralisation. However, oxygen uptake rates are dynamic, due to the diagenetic processes within the sediment, and can be spatially and temporally variable. Four benthic sites in the Celtic Sea, encompassing gradients of cohesive to permeable sediments, were sampled over four cruises to capture seasonal and spatial changes in oxygen dynamics. Total oxygen uptake (TOU) rates were measured through a suite of incubation experiments and oxygen microelectrode profiles were taken across all four benthic sites to provide the oxygen penetration depth and diffusive oxygen uptake (DOU) rates. The difference between TOU and DOU allowed for quantification of the fauna mediated oxygen uptake and diffusive uptake. High resolution measurements showed clear seasonal and spatial trends, with higher oxygen uptake rates measured in cohesive sediments compared to the permeable sediment. The significant differences in oxygen dynamics between the sediment types were consistent between seasons, with increasing oxygen consumption during and after the phytoplankton bloom. Carbon mineralisation in shelf sediments is strongly influenced by sediment type and seasonality.

FerryBox and databuoy measurements of plankton blooms

Two autonomous systems were used to record plankton blooms with reduced aliasing of timing and peak biomass in two different hypernutrified systems—a “FerryBox” and a “SmartBuoy”. Temperature, salinity, fluorescence, turbidity and position were measured on the ferry, plus nutrients on the SmartBuoys. A significant spring bloom occurred at both sites in 2001. This is a typical pattern of bloom development for the two sites and can be related to environmental conditions (nutrients, light and net heat flux). Chloro- phyll levels, however, remained low at both sites in 2002. This atypical year challenges our understanding of these systems as tidal and weather conditions were similar to 2001. The low values may be due to a combination of storm events at the end of 2001 and low winter nutrient values in 2002.