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.