Processes influencing phytoplankton growth and primary production in shallow temperate estuary Christchurch Harbour, United Kingdom

The aim of this research project was to identify the factors controlling the phytoplankton community and primary production in the shallow temperate estuary of Christchurch Harbour and the two river systems flowing into the estuary. Christchurch Harbour is a small shallow micro-tidal enclosed estuary situated on the south coast of England. It is fed by the rivers Stour and Hampshire Avon, and exchanges with coastal waters through a narrow channel at Mudeford. An intensive programme of monitoring both the water quality and phytoplankton communities was undertaken at weekly intervals from April 2013 to April 2014 at the lowest river gauging stations on the Hampshire Avon at Knapp Mill and the Stour at Throop plus a further station at Iford Bridge just above the tidal limit of the estuary. In addition a similar set of measurements were made on the same dates at the entrance to the estuary at Mudeford Quay at low tide. During Spring/Summer 2014 eight estuarine surveys were conducted at fortnightly intervals with measurements of water quality, phytoplankton abundance, and production made at six stations along a transect throughout the estuary.The riverine phytoplankton community in terms of carbon biomass and accessory pigments displayed a distinctive pattern of seasonal succession. A diatom group maximum was observed in spring and chlorophyte peak in summer. The nano-sized diatom (2.0 – 20.0 ?m), Stephanodiscus sp., dominated the phytoplankton assemblage, reaching 4.4 × 104 cells mL-1 and a chlorophyll concentration of 98.8 ?g L-1 on the Stour river during spring when river discharge had reduced following the winter flow peak. The summer chlorophyte bloom was composed of Chlamydomonas spp., reaching 7.9 × 104 cells mL-1 and followed the diatom spring bloom. Multivariate analysis revealed that water temperature, river discharge, silicate, and phosphate concentration were major factors controlling phytoplankton carbon biomass at all the river study sites.At Mudeford Quay, inorganic nutrient concentrations (nitrate, phosphate, and silicate) were generally low during periods of reduced river discharge, but increased during the winter high river flow periods. A chlorophyll a maximum was observed during the late spring and decreased during the autumn and winter similar to conditions on the rivers at Throop and Knapp Mill. Phytoplankton carbon biomass and accessory pigment concentration displayed a similar pattern. Diatoms dominated the phytoplankton biomass and community throughout most of the sampling period and were inversely correlated to silicate concentrations. The dinoflagellate Kryptoperidinium foliaceum was observed in high abundance during summer months at high salinity values and the freshwater diatom Stephanodiscus spp. dominated during the spring.During the estuarine transect surveys conducted over high tide, high chlorophyll ‘bloom’ events (chlorophyll up to 98 ?g L-1) were detected in the middle of the estuary in waters of salinity values over 30. Reduced river discharge in summer months led to an increase in higher salinity water in the mid estuary with these associated peaks in phytoplankton abundance. Different populations of estuarine phytoplankton were observed over the course of the summer with dinoflagellate blooms dominated by K. foliaceum, occurring in the mid-estuary followed by Cryptomonas spp. blooms. Multivariate analysis revealed that irradiance attenuation coefficient (k), salinity, oxygen saturation, temperature, nitrate, and silicate were the major factors controlling phytoplankton carbon biomass during the transect surveys. The results of the present study have provided an improved understanding of the factors controlling the production and distribution of estuarine phytoplankton communities in the shallow temperate, Christchurch Harbour estuary