Review of historical data and interpretation
The Fleet Study Group (FSG) was established in
1975 in recognition of the view that management
and protection of the Fleet lagoon and Chesil Bank
could best be achieved from a well informed position.
The FSG has a diverse membership of academic and
scientific practitioners, land managers and others.
Its aim is to encourage research into all aspects
of the Fleet (and more recently Portland Harbour),
although until recently the emphasis has been towards
biological, natural history, palaeoenvironmental
and physiographical studies.
An accessible archive of work carried out by members
of the Fleet Study Group and others is held at Weymouth
College Library, Dorset. It includes over 250 reports,
journal articles, letters, and other items of relevance
to all types of studies on the Fleet from 1933 to
present. The archive has been reviewed as part of
the current project in order to extract any relevant
information on current or past water quality, particularly
in relation to nutrients. Other current and recent
studies have also been reviewed as part of the project.
A summary of the archive information in relation
to understanding past and present water quality
of the Fleet is presented here.
Information in the FSG archive concerns primarily
the biological and historical interest of the Fleet.
Few references contain any information on water
quality, and those which do have very limited data
relating to spot samples of a few sites, and none
have reliable data on nutrients. There have been
no comprehensive surveys of water quality in the
Fleet prior to the Environment Agency investigations
from 1996 onwards (see following report sections).
Limited data on nutrient content of sewage effluent
inputs to the Fleet exists from 1990 to present.
It has, therefore, not been possible to identify
the past nutrient status of the Fleet, nor to identify
any reliable indicators of an increase or decrease
in nutrient levels with time, due to lack of data.
Data exist for temperature and salinity in the Fleet
where measurements were made at the same time as
biological studies. Review of other recent reports
supplied by English Nature (several still in early
draft form) which are not yet in the FSG archive
has also been carried out, including the thesis
(John, 1995) from which the following information
on phytoplankton and nutrients was extracted. A
list of references from the FSG archive which might
have had information of relevance to water quality
have been reviewed, and are listed and summarised
in Annex D.
John (1995) measured salinity and temperature,
and took samples for analysis for nutrients and
plant pigments, together with enumeration of plankton
species. Samples were taken during July-August 1995
from eight sites along the length of the Fleet.
Two algal blooms occurred at Abbotsbury during the
sampling period, the first at the end of July composed
of the dinoflagellate Oxyrrhis sp., the second
towards the end of August of the dinoflagellate
Glenodinium foliaceum, which caused red colouration
of the water. Elevated numbers of Oxyrrhis
sp. were observed in samples from further eastwards
to Moonfleet and Chickerell on days following the
first bloom. The second bloom occurred on the last
day of sampling at Abbotsbury, with high numbers
of Glenodinium foliaceum also detected in
the Clouds Hill sample. Lower numbers of Glenodinium
foliaceum were recorded from samples taken eastwards
in the Fleet as far as Chickerell Hive Point. Neither
of these two dinoflagellates were detected in samples
from the Narrows or further eastwards on any sampling
occasion, and neither are known to be toxic or produce
toxins which might harm fish or animals.
Measurements of plant pigments (including chlorophyll
a) in the samples reflected the concentrations of
phytoplankton found in the samples, with the exception
of the bloom of Oxyrrhis sp., as this organism
is not pigmented. On the day of the second bloom,
a tributary entering the Fleet at Abbotsbury Swannery
(referred to as Mill Stream (Abbey Barn) for EA
sampling) was also sampled, and found to contain
high numbers of single celled blue green algae.
Measurements of nutrient content of the water column
indicated generally average concentrations for brackish
waters of nitrate, phosphate and ammonium along
the Fleet, ranging from 0-10 µM.
However, peaks of nitrate were observed on one day
at Abbotsbury and Clouds Hill (25 and 6 µM
respectively, equivalent to 350 and 84 µg/l-N),
on another day at Langton Herring (9 µM,
or 126 µg/l-N) when dinoflagellates
were found in the plankton samples, and again at
Abbotsbury (10 µM, or 140 µg/l-N)
on the last day of sampling when the bloom of Glenodinium
foliaceum occurred. Extremely high levels of
nitrate were also detected in the stream water on
the day of the Glenodinium bloom, at a concentration
of 266 µM (3724 µg/l-N),
the concentration decreasing by an order of magnitude
in the short distance from the stream to the Abbotsbury
Embayment itself. Regression analysis indicated
that the elevated total inorganic nitrogen levels
observed at Abbotsbury were significantly correlated
with the increased population density of Glenodinium
foliaceum observed during the bloom in August.
An attempt was made to determine if phytoplankton
populations were nutrient limited or not, by comparison
of the ratios of different plant pigments (carotenoids
and chlorophyll a). However, the ratio varied considerably,
and did not correspond to variations in nutrient
content of the water. The variation was attributed
to the fact that the populations were of mixed species
of phytoplankton, so no clear conclusions could
be drawn from the comparison.
Whilst there are few references which contain information
on water quality, several raise concerns about possible
impacts on the Fleet including eutrophication, e.g.
Elton (1991), Holmes (1983), John (1995).