Propuesta

Elevated spring and summer levels of chlorophyll a are one of the primary symptoms of increased nutrient inputs to estuarine waters and as such are another response variable measurement. Levels of chlorophyll would be expected to increase in spring due to the natural spring bloom. It is pronounced or persistent blooms which cause concern. Chlorophyll a is the molecule mediating photosynthesis in almost all green plants, including phytoplankton. Rapid proliferation or blooms of phytoplankton, as reflected in elevated chlorophyll a levels, can occur throughout the ocean but are typically associated with temperate coastal and estuarine waters, notably in restricted exchange environments such as estuaries. During winter months growth of phytoplankton populations are at a minimum because of reduced temperature, light availability, and water column stability, and chlorophyll-a levels generally remain low. Monitoring of chlorophyll a is undertaken more often in spring and summer

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months when estuarine concentrations may exceed 50-80µg l-1, under optimum growing conditions (Monbet 1992).

In the UK, the indicator (mean) value for suspected eutrophic conditions is set at 10µg l-1 chlorophyll a (Dong et al., 2000; also DoE standard, [EA, 2001c]). Values greater than this, coupled with cell densities of 5 x 105 cells l-1 (500 cells ml-1) are considered indicative of phytoplankton blooms (EA, 1997).

There are indications that phytoplankton blooms have occurred in the Exe Estuary. In the upper estuary, up to 60µg l-1 chlorophyll a were recorded in 1987, whereas levels of 0.5 – 1.5µg l-1 were found in the inflowing river water and the sea water at the mouth of the estuary (Atkins, 1988).

Algal blooms were also observed in the estuary during the summer months of 1988 and 1999 but were not sustained throughout the summer (EA, 2001b). Algal species recorded in high numbers included Naviculoid sp., Nitzschia sp. and Chaetoceros; species often associated with brown surface scum. A large bloom of Nitzschia closterium was observed at Lower Wear in August 1998, and a large diatom bloom which included Chaetoceros occurred in the estuary during July 1999. Cell counts were highest in 1998, and on spring tides (Turf Locks and mid-estuary) in both years. On neap tides, chlorophyll a measurements were high but corresponding cell counts low, indicating the presence of dead or dying algae. Figure 53 shows the mean, maximum and minimum values of chlorophyll a concentrations recorded during the period 1998-1999. The maximum concentration (101µg l-1) occurred at Turf Locks. With the exception of Cockwood, mean chlorophyll a concentrations recorded exceeded the UK indicator mean for suspected eutrophic conditions.

80 82 82 79 0 20 40 60 80 100 120

Lower Wear Turf Locks Mid-estuary Cockwood

C h lo rophy ll -a µ g l -1 Chlorophyll-a Threshold value

Figure 53. Mean values for chlorophyll a in tidal waters of the Exe Estuary

1998-2000; Figures above bars refer to number of samples. Error bars represent

min and max concentrations (from EA, 2001b). Threshold value = 10µg l-1 _{- (mean) value}

for suspected eutrophic conditions (Dong et al., 2000); also DOE standard (EA, 2001b).

years. Data are mainly for April-September, although measurements taken in the winter months (Nov, Feb) are included for some sites. The UK indicator value for suspected eutrophic conditions of 10µg l-1 chlorophyll a (Dong et al., 2000) was exceeded at both Turf Locks and Mid-estuary during 2001 (mean 13.6 and 11.4µg l-1, respectively) with maximum concentrations (58 and 45µg l-1) occurring in April.

Chlorophyll-a in tidal waters C hl or ophyl l- a µg/l itr e 0 10 20 30 40 50 60

Lower Wear Turf Locks Mid-estuary Cockwood

Figure 54. Chlorophyll a in tidal waters of the Exe Estuary. Data source EA. Data is for 2001.

On a longer temporal scale, concentrations have been increasing over the past decade, this trend is exemplified in figure 55 showing measurements in the middle reaches of the estuary taken at Turf Locks. Temporal trends generally show a seasonal pattern for chlorophyll a in tidal waters, with the expected spring and summer blooms. The most pronounced of these was during July 1999 when elevated concentrations were recorded at most sites, up to 101µg l-1 at Turf. An unusual winter bloom is indicated by high concentrations of up to 44µg l-1 recorded at Turf during November 1990. These higher winter levels indicate that potentially eutrophic conditions existed in the estuary more than 10 years ago.

Exe Estuary at Turf

0 20 40 60 80 100 120

Dec-88 May-90 Sep-91 Jan-93 Jun-94 Oct-95 Mar-97 Jul-98 Dec-99 Apr-01 Sep-02

C h lo roph yll_ a µ g /L

Figure 55. Temporal variations for chlorophyll a in tidal waters of the Exe Estuary at Turf Locks. Data source EA.

Recent (2001) values for chlorophyll a in freshwaters entering the estuary are summarised in figure 56. Highest concentrations are recorded in the Exe at Trews

Weir with a maximum of 18.3µg l-1 in June (presumably freshwater phytoplankton species). Elevated levels of freshwaters plankton may lead to problems following die- off (release of DOC) as they enter more saline waters in any great numbers.

Chlorophyll-a in freshwater C hl or ophyl l- a µ g/ lit re 0 5 10 15 20

Exe Thorverton Weir Exe Trews Weir Grindle Brook

Figure 56. Mean annual values for chlorophyll a in freshwaters feeding the Exe Estuary. Data source EA. Data is for 2001.

There have been several complaints by members of the public, concerning the presence of brown foams and scums, indicative of algal blooms, principally in the Topsham and Starcross areas. One incident was investigated and the scum analysed for chemicals and algae in an attempt to establish its source but results were inconclusive (EA, 2001b). Increasingly, therefore, it is important to distinguish between natural blooms and those caused by “artificial” causes, typified by elevated and prolonged spring and summer levels of chlorophyll a. It is also important to try and predict under what circumstances nuisance blooms will occur.

Overall, data for chlorophyll a indicate regular phytoplankton blooms which may be increasing in intensity, and that these are probably related to elevated nitrate and phosphate concentrations. Rigorous monitoring would seem advisable to ensure that this trend does not escalate in to nuisance proportions.

NB. It is generally assumed that an N:P ratio of 10:1 is ideal for plant growth. At N:P ratios >10:1 (mainly in freshwater), P is thought to be limiting, and at N:P ratios < 10:1 (mainly in seawater) N is thought to be limiting. However, there are 3 large coastal areas in the UK where P may be limiting – from the Solent to Dartmouth; around the Severn from Padstow to Oxwich, and from the Humber to Essex (Parr et al,, 1999).