Investigation of the environmental control on the phytoplankton and bacterioplankton in two contrasting temperate estuaries

Frequent measurements of the physical‐chemical parameters and biologicalcomponents of estuaries are key for assessing the ecological status of thesetransitional waters. Little is known about the microbial community composition oftwo temperate South Coast UK estuaries, Southampton Water and ChristchurchHarbour (Mudeford Quay). The aim of this research project was to investigate howchanges in the abundance and dynamics of the dominant phylogenetic heterotrophicbacteria populations relate to major physical‐chemical parameters duringphytoplankton bloom periods in the spring and summer months in these twoestuaries.During 2013 in Southampton Water, the spring phytoplankton bloom occurred whenthe water temperature was below 10°C whereas in Christchurch Harbour it occurredat 14°C, and in the following years in Southampton Water at 14 °C and 15 °C, in 2014and 2015, respectively. The spring bloom chlorophyll a concentrations inSouthampton Water never exceeded 10 μg L‐1 in all three years while in ChristchurchHarbour a major peak in spring 2013 reached 44 μg L‐1. Surface salinity inSouthampton Water showed a narrow range of 27‐33 whereas, at Mudeford Quay atthe entrance to Christchurch Harbour a much larger range was detected of 1.3‐22.The concentration of inorganic nutrients detected between the two estuaries fornitrate, phosphate and silicate had a much higher range in Christchurch Harbourcompared to Southampton Water reflecting the contribution from different riversources. Identifying the biological components the influence of the physical‐chemicalcontrols affecting their dynamics and succession. The phytoplankton community inSouthampton Water was assessed from HPLC pigment analysis coupled withmicroscopic counts, and indicated diatoms (Skeletonema sp., Thalassiosira sp., andChaetoceros sp.) dominated the spring bloom and dinoflagellates dominated summerbloom with major species Scripsiella sp. and Prorocentrum sp.Nucleic acid staining (DAPI and SYBR Green I) was applied to 1% (PFA) preservedwater samples for total enumeration of bacterioplankton. A cytosense flowcytometry slightly overestimated the concentration of bacteria compared to DAPIcell counts determined using a fluorescent microscope mainly during thephytoplankton spring and summer blooms but overall with significant correlationbetween the two methods for Southampton Water and Christchurch Harbour (r =0.87, r = 0.85, p <0.0001, n = 32 respectively). Fluorescence in situ hybridization(FISH) with oligonucleotide probes was used to determine the abundance anddominance of various heterotrophic bacteria groups in samples from both estuariesand theses related to environmental conditions. Betaproteobacteria showed a strongnegative significant correlation with salinity (r = ‐0.95, p <0.0001, n = 29) in the twoestuaries indicating they favour fresh water systems. Alpha‐, andGammaproteobacteria were detected with variable significant correlation withtemperature and salinity. However, only a moderate correlation was observedbetween the phylogenetic groups and Chl‐a concentrations highlighting the fact thatheterotrophic bacteria may utilize organic carbon from other sources in theestuarine system. A principal component analysis (PCA), indicated temperature tobe the most influence on bacteria domain levels (Eubacteria and Euryarchaea). Whereas, at phylogenetic class level proteobacterial phyla, salinity and Chl‐a werethe most influence on their abundance and succession.