The issues surrounding the potential effects of nutrients, including phosphorus, in relation to nature conservation in estuaries and coastal waters have been reviewed by Parr and Wheeler (1996), Scott et al (1999) and Parr et al (1999). The reader is referred to these documents for further details.

Estuaries are the most difficult of all waterbodies on which to undertake source apportionment studies, although useful indicators of the relative importance of land-derived sources are provided by national studies. For example, the SDIA (1989) proposed that 20-25% of the phosphorus (P) in UK rivers is detergent-derived, with a similar proportion from human waste (faecal matter) and about 45% from agriculture. Giving more detail, the SDIA later attributed 3% of surface water phosphorus as being derived from direct industrial charges, 53% from sewage effluent, 25% from livestock, 2% from silage losses and 17% from soil run-off. Morse et al. (1993) produced similar values, albeit derived into different sources. They attributed 7% of the phosphorus in UK surface waters to background sources, with 17% derived from fertiliser, 10% from industry, 23% from detergents, 29% from human sources and 35% from livestock.

Estimating the marine-derived P load to estuaries and coastal waters is very much more difficult, but it should be remembered that estuaries are short-lived features (in geological terms) which, as you are reading this, are in-filling with deposited particulate matter from the fresh and marine water inputs. Suspended particulate matter contains phosphorus, so for estuaries that are rapidly in-filling (predominantly due to deposition of marine-derived particulate matter), this may account for a large proportion of the phosphorus budget. Thus, Parr et al (1999) calculated land and atmosphere-derived P budgets for the three estuaries within the Penllyn a=r Sarnau candidate SAC (see table below), but based on sediment accumulation rates, predicted that P inputs to the estuaries may be doubled due to the load of suspended particulate matter from the marine environment.

The estimation of land and atmosphere-derived nutrient inputs is discussed by Scott et al (1999), but more useful information is provided by Johnes et al (1994) and Mainstone et al (1996). Parr et al (1998) provided a useful worked example of N and P nutrient budgets/source apportionment for the upper reaches of the River Avon, Hampshire - exactly the same procedures can be used for tidal waters.

Phosphorus is present in the aquatic environment in both inorganic and organic forms. The principal inorganic form is orthophosphate which can be measured as dissolved orthophosphate (or soluble reactive phosphate SRP) by measuring phosphate in samples that have been filtered through a 0.45 _m mesh or as total reactive phosphate (TRP) by measuring phosphate in unfiltered samples.

Much of the monitoring undertaken by the Environment Agency in England and Wales involves the measurement of TRP, while the National Monitoring Programme uses SRP for sites in estuaries and coastal waters of the UK (MPMMG 1998).

Mean total reactive phosphate (TRP) (unfiltered orthophosphate) values in English and Welsh coastal waters range from 0.007 to 0.165 mg P l^-1 (Parr et al 1999). Phosphorus was included in the proposed GQA scheme for nutrients in estuaries (Gunby et al 1995) which, as for nitrogen, utilises the estimated nutrient (TRP) concentration in freshwater as the classification statistic, according to the following class boundaries:

For some estuaries in England, this provides the range of values shown in the table below.

As the freshwater input to estuaries from large lowland rivers usually has an N:P ratio of >10, the water column, particularly at the freshwater end of the estuary, is more likely to be P- than N-limited, but it appears that saltmarshes are usually N-limited. Although UK coastal waters have conventionally been described as nitrogen-limited, available data suggest that there are three major regions of coastline which are phosphorus-limited (i.e. the TIN:TRP ratio is >10; see Parr et al 1999). These regions extend from north of the Humber to the Essex estuaries, from the Solent to Dartmouth and around the Severn coastline (from Padstow to Oxwich).

The phosphorus cycle in estuaries and coastal waters has been summarised in Scott et al (1999) and Parr et al (1999).

The principal form of phosphate is orthophosphate which is assimilated by algae and converted to organic phosphate. After death of the algal cell, organic phosphate is released and converted into phosphate in dissolved inorganic, particulate and organic forms. Phosphate is associated with suspended particles in low salinity and high dissolved oxygen situations and, consequently, the concentrations of phosphate in the turbidity maximum of estuaries are large and the sediments are a significant sink for phosphorus. Sediments can also be a considerable source of phosphorus if the sediment becomes depleted in oxygen because, under these conditions, phosphate becomes desorbed and diffuses into the water column.

The effects of non-toxic substances, such as phosphorus, on the marine environment can be sub-divided into direct effects (those organisms directly affected by changes in the concentrations of phosphorus) and secondary effects (those arising in the ecosystem as a result of the changes in the organisms directly affected).

The terms nutrient enrichment and hyper-nutrification are used to describe the increasing concentrations of nutrients, including phosphorus, in the aquatic environment but do not relate to the consequences or effects of the increasing nutrient levels. The term eutrophication has been defined by the Environment Agency (1998) as "the enrichment of waters by inorganic plant nutrients which results in the stimulation of an array of symptomatic changes. These include the increased production of algae and/or other aquatic plants, affecting the quality of the water and disturbing the balance of organisms present within it. Such changes may be undesirable and interfere with water uses." As such, it encompasses both the increasing nutrient levels and the resulting direct and indirect effects.

The principal direct effect of increasing phosphorus concentrations in estuaries and coastal waters is its contribution, along with nitrogen, to stimulating productivity of phytoplankton in areas where primary productivity is not limited by light availability. Parr et al (1999) suggested that certain parts of the English and Welsh coast (north of the Humber to the Essex estuaries, Solent to Dartmouth and around the Severn coastline from Padstow to Oxwich) may be P- rather than N-limited and, in these areas, the consequences of increased phosphorus concentrations are likely to be more important.

As much of the phosphorus in the tidal environment becomes bound to particulate matter, the sediment tends to be highly enriched with phosphorus. Thus, the addition of more phosphorus should make relatively little difference to ecological communities within the sediment.

The indirect effects of increasing phosphorus concentrations are associated with the effects of eutrophication and are described with respect to nitrogen in the section on nitrogen. Reference should also be made to Scott et al 1999.

Potential effects include:

• stimulation of phytoplankton growth in the water column of estuaries and coastal waters;
• perturbation of the plankton community, including zooplankton, other invertebrates and fish, as a result of repeated phytoplankton blooms with the potential to reduce biodiversity;
• increased fluctuation is dissolved oxygen concentrations in the water column during the growth phase of a bloom with the potential for sub-lethal and lethal effects on invertebrates and fish;
• potential for depletion of oxygen concentrations in the water column and sediments as a result of the die-off of phytoplankton blooms with the potential for sub-lethal and lethal effects on invertebrates and fish;
• contribution to increased turbidity in the water column and reduction in light availability to macroalgae and other aquatic plants growing in the photic zone;
• potential for severe degradation of the ecosystem with adverse consequences for sea birds and Annex II sea mammals.

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