Changes in sediment processes across the western Irish Sea front.

Sediment characteristics, sediment respiration (oxygen uptake and sulphate reduction) and sediment–water nutrient exchange, in conjunction with water column structure and phytoplankton biomass were measured at five stations across the western Irish Sea front in August 2000. The transition from thermally stratified (surface to bottom temperature difference of 2.3 °C) to isothermal water (14.3 °C) occurred over a distance of 13 km. The influence of the front on phytoplankton biomass was limited to a small region of elevated near surface chlorophyll (2.23 mg m−3; 50% > biomass in mixed waters). The front clearly marked the boundary between depositional sediments (silt/clays) with elevated sediment pigment levels (≈60 mg m−2) on the western side, to pigment impoverished

Phytoplankton community structure at contrasting sites in the Irish Sea: a modelling investigation

Factors controlling phytoplankton community structure were investigated at two sites in the western Irish Sea using an ecosystem model coupled to a one-dimensional physics model. The ecosystem model includes two compartments for phytoplankton (diatoms and non-diatoms) and zooplankton (meso- and microzooplankton), two nutrients (nitrogen and silicate) and a parameterisation of sediment dynamics. Diatoms were assigned higher maximum growth rates than non-diatoms in the model such that they are superior competitors, providing silicate is non-limiting. Resource limitation was formulated using a multiplicative model and temperature effects on biological processes were not included on account of a lack of a clear approach to do so. The model successfully reproduced seasonal cycles of primary production, chlorophyll and nutrients at both sites. The predicted dominance of diatoms during the summer at the coastal site was due to the supply of silicate from the sediment and top-down control of nondiatoms by microzooplankton grazing. Seasonal stratification at the offshore site prevented silicate of sedimentary origin from reaching surface waters, causing the mixed spring diatom-flagellate bloom to be replaced by non-diatoms with occasional diatom resurgence. Higher diatom abundance resulted in reduced grazing on microzooplankton by mesozooplankton and explains the higher biomass of microzooplankton at the coastal station. It is concluded that under conditions of nutrient re-supply, top-down grazing helps to control the balance between diatoms and non-diatoms. In deeper, stratified waters, there is only bottom-up control (nutrient limitation) of phytoplankton community structure

Relationships between suspended particulate material, light attenuation and Secchi depth in UK marine waters

Measurements of sub-surface light attenuation (Kd), Secchi depth and suspended particulate material (SPM) were made at 382 locations in transitional, coastal and offshore waters around the United Kingdom (hereafter UK) between August 2004 and December 2005. Data were analysed statistically in relation to a marine water typology characterised by differences in tidal range, mixing and salinity. There was a strong statistically significant linear relationship between SPM and Kd for the full data set. We show that slightly better results are obtained by fitting separate models to data from transitional waters and coastal and offshore waters combined. These linear models were used to predict Kd from SPM. Using a statistic (D) to quantify the error of prediction of Kd from SPM, we found an overall prediction error rate of 23.1%. Statistically significant linear relationships were also evident between the log of Secchi depth and the log of Kd in waters around the UK. Again, statistically significant improvements were obtained by fitting separate models to estuarine and combined coastal/offshore data – however, the prediction error was improved only marginally, from 31.6% to 29.7%. Prediction was poor in transitional waters (D = 39.5%) but relatively good in coastal/offshore waters (D = 26.9%).\ud \ud SPM data were extracted from long term monitoring data sites held by the UK Environment Agency. The appropriate linear models (estuarine or combined coastal/offshore) were applied to the SPM data to obtain representative Kd values from estuarine, coastal and offshore sites. Estuarine waters typically had higher concentrations of SPM (8.2–73.8 mg l−1) compared to coastal waters (3.0–24.1 mg l−1) and offshore waters (9.3 mg l−1). The higher SPM values in estuarine waters corresponded to higher values of Kd (0.8–5.6 m−1). Water types that were identified by large tidal ranges and exposure typically had the highest Kd ranges in both estuarine and coastal waters. In terms of susceptibility to eutrophication, large macrotidal, well mixed estuarine waters, such as the Thames embayment and the Humber estuary were identified at least risk from eutrophic conditions due to light-limiting conditions of the water type

Assessing trends in nutrient concentrations in coastal shelf seas: a case study in the Irish Sea

Single point time-series data collected in the Irish Sea since 1954 by Allen et al. (1998) have been re-evaluated using more recent data and areal winter surveys of the Celtic and Irish Seas. Survey data illustrate the heterogeneity of nutrient distributions with N (?28 M) and P (2·0 M) enrichment of the eastern Irish Sea relative to the western Irish Sea (7–8 M N and 0·7 M P). Salinity nutrient relationships demonstrate distinct regional differences throughout the two seas. Trends in the Isle of Man time-series are unlikely to have resulted from changes in analytical procedures and personnel. Concerns over data quality cannot be resolved and there are no independent data to validate the time-series. Including more recent data shows P has declined since the late 1980s and N concentrations have remained stable since the mid 1970s. The absence of a Si trend is consistent with limited anthropogenic influence on riverine Si concentrations. Trends in N and P are consistent with changes in riverine concentrations of these two nutrients and the biogeochemical processes controlling their cycling in shelf waters. Denitrification is the most likely reason for offshore Irish Sea concentrations of N being lower than expected

Estimating the diffuse attenuation coefficient from optically active constituents in UK marine waters

We report a study of the attenuation of submarine Photosynthetically Active Radiation (PAR) in relation\ud to the concentrations of Optically Active Constituents (OACs) in a range of water types around the United\ud Kingdom. 408 locations were visited between August 2004 and December 2005. The diffuse attenuation\ud coefficient (Kd) was estimated from profiles of downwelling PAR. Concentrations of Suspended Particulate\ud Matter (SPM) were measured gravimetrically and concentrations of phytoplankton chlorophyll\ud (chl) were measured by fluorometrically. Chromophoric Dissolved Organic Matter (CDOM) was measured\ud either by fluorescence or as its proxy, salinity.\ud Several empirical models for Kd as a function of SPM, chlorophyll and CDOM were fitted to the data set. It\ud was found that including all three explanatory variables (CDOM, chlorophyll and SPM) gave a slightly\ud better fit for coastal and offshore waters, whereas a fit based only on SPM and chlorophyll worked well\ud for transitional (estuarine) waters. The use of SPM as a single predictor of Kd in all water types resulted in\ud only 3% loss of accuracy.\ud The effect of seasonal variations in the light climate and the OACs was investigated with high frequency\ud data from moorings in the Thames embayment and Liverpool Bay. Kd was estimated from data recorded\ud from pairs of vertically separated PAR sensors. Using the empirical models to estimate Kd from these OACs showed that reliable estimates of attenuation could be made throughout the year, with some scatter of estimated Kd about observed Kd during the growing season. The reliability of these findings was validated by non-linearly fitting of a mechanistic model, based on semi-intrinsic optical parameters, to the spatial data set. Estimated values of absorption cross-section and scattering cross-section were in good agreement with the literature, and help to justify parameter values obtained from the empirical models

The Irish Sea: Is it eutrophic?

The question of whether the Irish Sea is eutrophic is addressed by reviewing the evidence for anthropogenic nutrient enrichment, elevated phytoplankton production and biomass and undesirable disturbance in the context of the EU and OSPAR definitions of eutrophication. Winter concentrations of dissolved available inorganic phosphate (DAIP), nitrogen (DAIN as nitrate and nitrite) and silicate (Si) in coastal waters and concentrations of DAIP and Si in offshore waters of the Irish Sea are elevated relative to winter Celtic Sea shelf break concentrations (0.5 μM DAIP, 7.7 μM DAIN and 2.7 μM Si). Significant, negative nutrient salinity relationships and analysis of the Isle of Man nutrient time-series indicate that the elevated Irish Sea levels of DAIP and DAIN are the result of anthropogenic enrichment with highest concentrations (≈2.0 μM DAIP, 30 μM DAIN and 17 μM Si) measured in near shore eastern Irish Sea waters.\ud \ud Summer levels of phytoplankton chlorophyll (Chl) range from <0.1 to 11.4 mg m−3 (mean: 3.4 mg m−3) and from <0.1 to 16.4 mg m−3 (mean: 2.2 mg m−3) in coastal and offshore waters of the western Irish Sea, respectively. Offshore eastern Irish Sea summer chlorophyll levels range from 0.3 to 3.8 mg m−3 (mean: 1.8 mg m−3). Higher levels of spring (up to 43.9 mg m−3) and summer (up to 22.7 mg m−3) biomass in Liverpool Bay are attributed to nutrient enrichment. Estimates of spring and summer production in different regions of the Irish Sea are ≤194 g C m−2.\ud \ud The absence of: (a) oxygen depletion in near shore and open waters of the Irish Sea (except the seasonally isolated western Irish Sea bottom water); (b) trends in the frequency of Phaeocystis spp. blooms and occurrence of toxin producing algae; and (c) changes in the dominant life form of pelagic primary producers, point to a lack of undesirable disturbance and hence argue against anthropogenic eutrophication in the Irish Sea. This conclusion is discussed in the context of future trends in anthropogenic nutrient inputs