Carolyn Harris

Concentrations of suspended chlorophyll in the tidal Yorkshire Ouse and Humber Estuary

Measurements of salinity, suspended particulate matter, chlorophyll a and phaeopigments in the tidal reaches of the Yorkshire Ouse and Humber Estuary, UK, are presented for the period March 1994–March 1995. Chlorophyll a concentrations were<3 μg l−1 throughout the tidal Ouse and Humber during March and April 1994. Generally, concentrations were also low in the non-tidal Yorkshire Ouse (<4 μg l−1). Increases in chlorophyll a in the non-tidal river during May–August corresponded with increased concentrations in the tidal river, with highest concentrations occurring at the lowest observed salinities. Large declines in chlorophyll a concentrations, from 60 to<13 μg l−1, occurred along a transect from the non-tidal river to the tidal river during summer, which probably resulted from a combination of net negative growth rates and mortality in the extremely turbid receiving waters. Chlorophyll a concentration decreased to<10 μg l−1 (but usually<5 μg l−1) in the non-tidal river during autumn and was less than 1 μg l−1 throughout much of the winter. Concentrations were also at a minimum within the tidal river and Humber. However, a noticeable feature was a local maximum of chlorophyll a at the interface between fresh and brackish waters, which was associated with the zone of higher turbidity.

Concentrations of dissolved nutrients in the tidal Yorkshire Ouse River and Humber Estuary

Abstract Measurements of dissolved nitrate, silicate, phosphate, nitrite and ammonium in the tidal reaches of the Yorkshire Ouse and Humber Estuary, UK, are presented for the period March 1994 to March 1995. The data and their spatial trends are consistent with independent measurements derived from both freshwater and coastal measurements. Nitrate was typically less than about 550 μM. Concentrations were relatively low in the outer Humber and coastal zone and increased strongly up-estuary, reaching a mid-estuarine peak before decreasing to the lower concentrations that characterized the non-tidal river. The nitrate peak was well defined during summer and early autumn, when a strong maximum in turbidity occurred. Silicate distributions were similar to nitrate but with mid-estuarine maxima in the range 90–260 μM. These maxima were closely associated with the high turbidity zone during summer and autumn. Phosphate was dominated by freshwater inputs during spring through late summer, but exhibited a mid-estuarine peak between 5 and 10 μM and located several km up-estuary of the freshwater-saltwater interface (FSI) during the rest of the year. A pronounced peak in nitrite, between 9 and 12 μM and associated with the high turbidity zone, was observed up-estuary of the FSI during summer and autumn followed by a sharp drop to low concentrations. This nitrite depletion zone was located down-estuary of the high turbidity zone. Twin, mid-estuarine ammonium maxima were observed during winter. The up-estuary maxima (∼35 μM) were located about 30-km up-estuary of the FSI and the higher turbidity zone. Nutrient distributions are discussed in terms of physical and local, source-sink processes, and the various inputs from freshwater tributaries and wastewater discharges.

Both respiration and photosynthesis determine the scaling of plankton metabolism in the oligotrophic ocean.

Despite its importance to ocean–climate interactions, the metabolic state of the oligotrophic ocean has remained controversial for >15 years. Positions in the debate are that it is either hetero- or autotrophic, which suggests either substantial unaccounted for organic matter inputs, or that all available photosynthesis (P) estimations (including 14C) are biased. Here we show the existence of systematic differences in the metabolic state of the North (heterotrophic) and South (autotrophic) Atlantic oligotrophic gyres, resulting from differences in both P and respiration (R). The oligotrophic ocean is neither auto- nor heterotrophic, but functionally diverse. Our results show that the scaling of plankton metabolism by generalized P:R relationships that has sustained the debate is biased, and indicate that the variability of R, and not only of P, needs to be considered in regional estimations of the oceans metabolic state.

Seasonal iron depletion in temperate shelf seas

Our study followed the seasonal cycling of soluble (SFe), colloidal (CFe), dissolved (DFe), total dissolvable (TDFe), labile particulate (LPFe) and total particulate (TPFe) iron in the Celtic Sea (NE Atlantic Ocean). Preferential uptake of SFe occurred during the spring bloom, preceding the removal of CFe. Uptake and export of Fe during the spring bloom, coupled with a reduction in vertical exchange, led to Fe deplete surface waters (<0.2 nM DFe; 0.11 nM LPFe, 0.45 nM TDFe, 1.84 nM TPFe) during summer stratification. Below the seasonal thermocline, DFe concentrations increased from spring to autumn, mirroring NO3- and consistent with supply from remineralised sinking organic material, and cycled independently of particulate Fe over seasonal timescales. These results demonstrate that summer Fe availability is comparable to the seasonally Fe limited Ross Sea shelf, and therefore is likely low enough to affect phytoplankton growth and species composition.

Corrigendum: Both respiration and photosynthesis determine the scaling of plankton metabolism in the oligotrophic ocean.

Nature Communications 6:6961 doi: 10.1038/ncomms7961 (2015); Published April242015; Updated 2016 The original version of this Article failed to fully credit the use of the Ocean Data View software in figure 3, which appears below: Schlitzer, R., Ocean Data View, http://odv.awi.de, 2016.