Bottom mixed layer oxygen dynamics in the Celtic Sea

The seasonally stratified continental shelf seas are highly productive, economically important environments which are under considerable pressure from human activity. Global dissolved oxygen concentrations have shown rapid reductions in response to anthropogenic forcing since at least the middle of the twentieth century. Oxygen consumption is at the same time linked to the cycling of atmospheric carbon, with oxygen being a proxy for carbon remineralisation and the release of CO2. In the seasonally stratified seas the bottom mixed layer (BML) is partially isolated from the atmosphere and is thus controlled by interplay between oxygen consumption processes, vertical and horizontal advection. Oxygen consumption rates can be both spatially and temporally dynamic, but these dynamics are often missed with incubation based techniques. Here we adopt a Bayesian approach to determining total BML oxygen consumption rates from a high resolution oxygen time-series. This incorporates both our knowledge and our uncertainty of the various processes which control the oxygen inventory. Total BML rates integrate both processes in the water column and at the sediment interface. These observations span the stratified period of the Celtic Sea and across both sandy and muddy sediment types. We show how horizontal advection, tidal forcing and vertical mixing together control the bottom mixed layer oxygen concentrations at various times over the stratified period. Our muddy-sand site shows cyclic spring-neap mediated changes in oxygen consumption driven by the frequent resuspension or ventilation of the seabed. We see evidence for prolonged periods of increased vertical mixing which provide the ventilation necessary to support the high rates of consumption observed

Mechanisms for a nutrient-conserving carbon pump in a seasonally stratified, temperate continental shelf sea

Continental shelf seas may have a significant role in oceanic uptake and storage of carbon dioxide (CO2) from the atmosphere, through a ‘continental shelf pump’ mechanism. The northwest European continental shelf, in particular the Celtic Sea (50°N 8°W), was the target of extensive biogeochemical sampling from March 2014 to September 2015, as part of the UK Shelf Sea Biogeochemistry research programme (UK-SSB). Here, we use the UK-SSB carbonate chemistry and macronutrient measurements to investigate the biogeochemical seasonality in this temperate, seasonally stratified system. Following the onset of stratification, near-surface biological primary production during spring and summer removed dissolved inorganic carbon and nutrients, and a fraction of the sinking particulate organic matter was subsequently remineralised beneath the thermocline. Water column inventories of these variables throughout 1.5 seasonal cycles, corrected for air-sea CO2 exchange and sedimentary denitrification and anammox, isolated the combined effect of net community production (NCP) and remineralisation on the inorganic macronutrient inventories. Overall inorganic inventory changes suggested that a significant fraction (>50%) of the annual NCP of around 3 mol-C m–2 yr–1 appeared to be stored within a long-lived organic matter (OM) pool with a lifetime of several months or more. Moreover, transfers into and out of this pool appeared not to be in steady state over the one full seasonal cycle sampled. Accumulation of such a long-lived and potentially C-rich OM pool is suggested to be at least partially responsible for the estimated net air-to-sea CO2 flux of ∼1.3 mol-C m–2 yr–1 at our study site, while providing a mechanism through which a nutrient-conserving continental shelf pump for CO2 could potentially operate in this and other similar regions

A Tale of Four Seasons: Investigating the seasonality of physical structure and its biogeochemical responses in a temperate continental shelf sea

Due to their high biological productivity, continental shelf seas are significant sinks of anthropogenic carbon. To better understand the cycling of carbon within them, an accurate prediction of their vertical density stratification is required, as this is a critical control on the carbon drawdown. While the dominant controls on density structure are boundary driven mixing and seasonal heating, internal waves have been shown to play a small but critical role in both open ocean and shelf sea physical and biogeochemical cycles. Current knowledge on the spatial and temporal variability in internal mixing is however still severely limited. The aim of the work in this thesis was to develop new insight into the seasonal variability of physical controls on the vertical density structure and examine its biogeochemical responses in a temperate shelf sea. This thesis presents and examines new results that test the impact of boundary layer and internal wave forcing on stratification and vertical density structure in continental shelves. A new series of continuous measurements of full depth density and velocity structure, meteorological and wave forcing, surface nitrate and surface chlorophyll a spanning 17 months (March’14 − July’15) provide unprecedented coverage over a full seasonal cycle at a station 120 km north-east from the continental shelf break. Work within this thesis showed that the controls on vertical density structure at the mooring site were largely analogous to that of open-ocean environments with tidal mixing only playing a minor role. This result contrasts with the well-known tidally controlled frontal systems described by Simpson and Hunter (1974). Since a large proportion of continental shelf regions are away from tidal mixing fronts this result suggest the requirement for an adjusted third regime that bridges the gap between open-ocean environments and frontal regions, to accurately predict the vertical density structure within them. The long-term observations presented in this thesis reveal a seasonality within the internal wave energy, which suggest internal mixing varying relative to the seasonal cycle of stratification, represented by N². By investigating the representation of this seasonality by three commonly used internal wave parameterisations it was shown that each predicted a seasonality that directly contradicted that observed within the internal wave energy. It was suggested that the reason for this was most likely due to their failure to introduce the enhanced S² that is attributable to internal waves, which have been shown in this work to have a strong seasonal cycle with maximum energy levels during the summer. In an attempt to provide realistic scaling of turbulence an adjusted iteration of the MacKinnon and Gregg (2003a) scaling of turbulence was employed using an observed close relationship between N² and S² to introduce a state of marginal stability in the pycnocline, thus introducing a seasonally varying level of internal mixing that follows the observed seasonality in internal wave energy. Examining the biogeochemical response to the seasonal change in vertical density struc- ture also highlighted the importance of the autumn phytoplankton bloom within the annual cycle of primary production. By putting the autumn phytoplankton bloom within the context of the seasonal cycle it was shown that it has the potential to be as productive as the well-studied spring phytoplankton bloom and the summer sub-surface chlorophyll maximum and thus has the capacity to significantly contribute to the drawdown of atmospheric carbon dioxide

Seasonal changes in plankton respiration and bacterial metabolism in a temperate shelf sea

The seasonal variability of plankton metabolism indicates how much carbon is cycling within a system, as well as its capacity to store carbon or export organic matter and CO2 to the deep ocean. Seasonal variability between November 2014, April 2015 and July 2015 in plankton respiration and bacterial (Bacteria+Archaea) metabolism is reported for the upper and bottom mixing layers at two stations in the Celtic Sea, UK. Upper mixing layer (UML, >75 m in November, 41 - 70 m in April and ~50 m in July) depth-integrated plankton metabolism showed strong seasonal changes with a maximum in April for plankton respiration (1.2- to 2-fold greater compared to November and July, respectively) and in July for bacterial production (2-fold greater compared to November and April). However UML depth-integrated bacterial respiration was similar in November and April and 2-fold lower in July. The greater variability in bacterial production compared to bacterial respiration drove seasonal changes in bacterial growth efficiencies, which had maximum values of 89 % in July and minimum values of 5 % in November. Rates of respiration and gross primary production (14C-PP) also showed different seasonal patterns, resulting in seasonal changes in 14C-PP:CRO2 ratios. In April, the system was net autotrophic (14C-PP:CRO2 > 1), with a surplus of organic matter available for higher trophic levels and export, while in July balanced metabolism occurred (14C-PP:CRO2 = 1) due to an increase in plankton respiration and a decrease in gross primary production. Comparison of the UML and bottom mixing layer indicated that plankton respiration and bacterial production were higher (between 4 and 8-fold and 4 and 7-fold, respectively) in the UML than below. However, the rates of bacterial respiration were not statistically different (p > 0.05) between the two mixing layers in any of the three sampled seasons. These results highlight that, contrary to previous data from shelf seas, the production of CO2 by the plankton community in the UML, which is then available to degas to the atmosphere, is greater than the respiratory production of dissolved inorganic carbon in deeper waters, which may contribute to offshore export

Propagating uncertainty from physical and biogeochemical drivers through to top predators in dynamic Bayesian ecosystem models improves predictions

With the global rapid expansion of offshore renewable energies, there is an urgent need to assess and predict effects on marine species, habitats, and ecosystem functioning. Doing so will require dynamic, multitrophic, ecosystem-centric approaches coupled with oceanographic models that can allow for physical and/or biogeochemical indicators of marine ecosystem change to be included. However, in such coupled approaches, indicators carry uncertainties that can propagate and affect species higher up the trophic chain. Dynamic Bayesian networks (DBNs) are pragmatic approaches that probabilistically represent ecosystem-level interactions. They allow for uncertainties to be better estimated than mechanistic models that only account for expected values. In this study, we calculated variance as a measure of uncertainty from selected indicators and used them to build DBN models. A hidden variable was incorporated to model functional ecosystem change, where the underlying interactions dramatically change, following a disturbance. We wanted to assess whether propagating uncertainty into the modelling process affects the predictive accuracy of the models in the context of reconstructing the time series of the ecosystem dynamics. Model accuracy was improved for 60 % of the species once variance was added. The models were better in capturing the temporal inter-annual variability, once variance was calculated with a rolling window approach. The hidden variable successfully modelled previously identified ecosystem changes, however, now with the added uncertainty, the changes that implicated the ecosystem state were identified earlier in the time series. The results indicate that using DBNs is highly valuable as it gains accuracy with the addition of uncertainty

Dissolution Dominates Silica Cycling in a Shelf Sea Autumn Bloom

Autumn phytoplankton blooms represent key periods of production in temperate and high‐latitude seas. Biogenic silica (bSiO2) production, dissolution, and standing stocks were determined in the Celtic Sea (United Kingdom) during November 2014. Dissolution rates were in excess of bSiO2 production, indicating a net loss of bSiO2. Estimated diatom bSiO2 contributed ≤10% to total bSiO2, with detritalbSiO2 supportingrapidSicycling.Basedontheaveragebiomass‐specificdissolutionrate(0.2day−1), 3weekswouldbeneededtodissolve99%ofthebSiO2 present.NegativenetbSiO2 productionwasassociated with low‐light conditions (<4 E·m−2·day−1). Our observations imply that dissolution dominates Si cycling during autumn, with low‐light conditions also likely to influence Si cycling during winter and early spring

Shelf‐break upwelling and productivity over the North Kenya Banks: The importance of large‐scale ocean dynamics

The North Kenya Banks (NKBs) have recently emerged as a new frontier for food security and could become an economically important fishery for Kenya with improved resources providing better accessibility. Little research has been done on the mechanisms supporting high fish productivity over the NKBs with information on annual and interannual environmental variability lacking. Here we use a high‐resolution, global, biogeochemical ocean model with remote sensing observations to demonstrate that the ocean circulation exerts an important control on the productivity over the NKBs. During the Northeast Monsoon, which occurs from December to February, upwelling occurs along the Kenyan coast, which is topographically enhanced over the NKBs. Additionally, enhanced upwelling events, associated with widespread cool temperatures, elevated chlorophyll, nutrients, primary production, and phytoplankton biomass, can occur over this region. Eight such modeled events, characterized by primary production exceeding 1.3 g C/m−2/day, were found to occur during January or February from 1993–2015. Even though the upwelling is always rooted to the NKBs, the position, spatial extent, and intensity of the upwelling exhibit considerable interannual variability. The confluence zone between the Somali Current and East African Coastal Current (referred to as the Somali‐Zanzibar Confluence Zone) forms during the Northeast Monsoon and is highly variable. We present evidence that when the Somali‐Zanzibar Confluence Zone is positioned further south, it acts to enhance shelf‐edge upwelling and productivity over the NKBs. These findings provide the first indication of the environmental controls that need to be considered when developing plans for the sustainable exploitation of the NKB fishery

Sensitivity of shelf sea marine ecosystems to temporal resolution of meteorological forcing

Phytoplankton phenology and the length of the growing season have implications that cascade through trophic levels and ultimately impact the global carbon flux to the seafloor. Coupled hydrodynamic‐ecosystem models must accurately predict timing and duration of phytoplankton blooms in order to predict the impact of environmental change on ecosystem dynamics. Meteorological conditions, such as solar irradiance, air temperature and wind‐speed are known to strongly impact the timing of phytoplankton blooms. Here, we investigate the impact of degrading the temporal resolution of meteorological forcing (wind, surface pressure, air and dew point temperatures) from 1‐24 hours using a 1D coupled hydrodynamic‐ecosystem model at two contrasting shelf‐sea sites: one coastal intermediately stratified site (L4) and one offshore site with constant summer stratification (CCS). Higher temporal resolutions of meteorological forcing resulted in greater wind stress acting on the sea surface increasing water column turbulent kinetic energy. Consequently, the water column was stratified for a smaller proportion of the year producing a delayed onset of the spring phytoplankton bloom by up to 6 days, often earlier cessation of the autumn bloom, and shortened growing season of up to 23 days. Despite opposing trends in gross primary production between sites, a weakened microbial loop occurred with higher meteorological resolution due to reduced dissolved organic carbon production by phytoplankton caused by differences in resource limitation: light at CCS and nitrate at L4. Caution should be taken when comparing model runs with differing meteorological forcing resolutions. Recalibration of hydrodynamic‐ecosystem models may be required if meteorological resolution is upgraded

A paradigm for understanding whole ecosystem effects of offshore wind farms in shelf seas

We would like to thank Emma Ahart, David Bould, Constance Schéré, Marie Toulon, and Inne Withouck for comments on drafts. Also thanks to three anonymous reviewers and Howard Browman for constructive review.Peer reviewe

Interannual monsoon wind variability as a key driver of East African small pelagic fisheries

Small pelagic fisheries provide food security, livelihood support and economic stability for East African coastal communities—a region of least developed countries. Using remotely- sensed and field observations together with modelling, we address the biophysical drivers of this important resource. We show that annual variations of fisheries yield parallel those of chlorophyll-a (an index of phytoplankton biomass). While enhanced phytoplankton biomass during the Northeast monsoon is triggered by wind-driven upwelling, during the Southeast monsoon, it is driven by two current induced mechanisms: coastal “dynamic uplift” upwelling; and westward advection of nutrients. This biological response to the Southeast monsoon is greater than that to the Northeast monsoon. For years unaffected by strong El-Niño/La-Niña events, the Southeast monsoon wind strength over the south tropical Indian Ocean is the main driver of year-to-year variability. This has important implications for the predictability of fisheries yield, its response to climate change, policy and resource management