The following sections summarise water quality issues of concern for lagoons (other than salinity) in priority order. Note that there is more information on nutrient enrichment than other water quality issues.

Scott et al (1999) provide a useful overview of nutrients and related impacts in estuaries, much of which is relevant to lagoons.

It is evident that lagoons can be naturally rich in nutrients. In a review of lagoons (albeit of a wider definition than in the UK) from Europe and the Americas, it was found, in common with other near-shore brackish environments, that there is a summer phosphate maximum in contrast to the open sea (Nixon 1982).

There is some debate as to which nutrient is limiting in coastal waters (Taylor et al 1995, Cole et al 1999). Taylor et al (1995) undertook mesocosm experiments which indicated that the issue of nutrient limitation of lagoons is complex. Whilst their results suggested nitrogen limitation in coastal lagoons of the northeast United States, this depended on whether the entire lagoon system or plant communities within it were under consideration. They further concluded that lagoons will pass through successive shifts in limitation, ie a shift to phosphorus limitation following enrichment by nitrogen, and a concomitant shift in limitations on particular components of the system, i.e eelgrass and macroalgae shifting from nutrient to light limitation. Work in the Fleet reported herein did not indicate that either nitrogen or phosphorus was more limiting throughout most of the lagoon and that both may be limiting at different times of the year. It is apparent therefore that either nitrogen or phosphorus, or both nutrients, could be limiting in lagoons, depending on characteristics of the site, and that this may vary spatially, particularly in sites with a pronounced environmental gradient, or seasonally.

For the same reasons that they are naturally rich in nutrients, i.e. their restricted exchange with the sea and concomitant reduced flushing of dissolved or suspended materials, saline lagoons are also sensitive to nutrient enrichment. The fact that many lagoons may be naturally nutrient rich, even if only seasonally, may mean that relatively low additional inputs of nutrients could cause symptoms of eutrophication. There are a number of potential sources of inputs of dissolved and suspended materials including freshwater and seawater inflows, groundwater, adjacent land by runoff, and even airborne sources, as well as from direct anthropogenic sources.

A number of effects may result from nutrient enrichment in lagoons including:

• direct physiological or metabolic effects as evident in charophytes in particular (in relation to phosphorus) but also in seagrass Zostera (at high concentrations of nitrates) with apparent lower sensitivity in tasselweed Ruppia;
• increased growth of epiphytic algae, particularly on seagrass and macroalgae, and associated competition for light. The importance of this relationship cannot be overstressed in terms of the effects of nutrient enrichment, since one of the classic symptoms of eutrophication in marine and freshwater ecosystems is a large increase in epiphyte standing crop (Parr et al 1998);
• increased growth of blanketing or floating algae, e.g. Cladophora spp, and reduced light to lagoonal vegetation including charophytes, seagrass, tasselweeds and macroalgae;
• increased growth of ephemeral benthic algae and associated competition for light and space with other lagoonal vegetation;
• increased phytoplankton standing crop with various effects:
• in particular an increase in light attenuation and associated direct effects on lagoonal vegetation;
• effects on populations of invertebrates dependant on lagoonal vegetation. Bamber (1997) found distinct bias in types of lagoonal invertebrate communities present at different sites, in relation to the presence or absence of vegetation;
• lagoonal fauna, in common with other marine or freshwater fauna, are likely to be susceptible to the effects of harmful phytoplankton blooms. Fish in particular are susceptible to toxins produced by some phytoplankton, and to clogging of the gills when plankton are particularly dense in the water column;
• fish and benthic fauna and flora may be susceptible to reductions in oxygen content of the water when phytoplankton blooms or excessive algal growth decay.

Hodgkin and Birch (1982) provide a good example of the complexity of the relationship between lagoons and nutrient inputs and the vulnerability of coastal lagoons to eutrophication in a study of two shallow (2 metre) coastal lagoons (essentially an enclosed estuarine lagoonal system) in Western Australia. In that case, excessive growth and accumulation of benthic green algae (principally Cladophora species) and dense phytoplankton blooms occurred, causing various impacts including loss of seagrass. The increase in algal and phytoplankton productivity was attributed to a great increase in the input of nutrients, especially of phosphorus, as a result of the enhanced use of phosphorus based fertilisers in the agricultural catchment. High freshwater drainage inputs during winter caused the influx and small tidal range and, therefore, water exchange with the adjacent sea was limited. Whilst low temperatures and light conditions limited benthic algal growth during winter, the nutrients were retained in the system by diatom blooms and recycled to detrital sediment to become available to benthic algae and phytoplankton in summer. At that time the most promising approach for addressing the problem was to decrease phosphorus inputs by modifying fertilizer practice. In order to effect a solution in an acceptable time frame, a more radical management measure was suggested, i.e. opening a new connection with the sea to increase tidal exchange (Hodgkin & Birch 1986), and subsequently implemented (Hodgkin & Hamilton 1993).

Increased turbidity may be perceived as a potential problem for lagoon sites due to:

• direct increase in light attenuation, affecting photosynthesis by eelgrasses and tasselweeds, as well as macroalgae;
• possible smothering or inhibition of feeding of fragile specialist lagoonal species (such as the starlet anemone Nematostella vectensis) by suspended inorganic matter settling out of the water column.

Turbidity may be caused by biotic growth (plankton) in the water column, or due to presence of suspended particles.

In some areas, e.g. peat areas in the catchment of some sites in Scotland, water colour will be a factor influencing light penetration and, therefore, photosynthesis of plants and algae. Whilst the vegetation at lagoon sites within these areas will be adapted to such conditions, consideration should be given to any factors that may change water colour and affect light penetration, e.g. increased peat erosion.

No examples were found of saline lagoons being affected by toxic contamination, whether by non-synthetics such as hydrocarbons or synthetics such as pesticides. However, such systems are potentially highly sensitive to such inputs given their poor to moderately poor flushing characteristics, and those adjacent to urban or industrial development are potentially most vulnerable. Studies from outside lagoons suggest the following toxic contaminants may be of concern:

• heavy metals and organic substances affecting physiology of seagrass Zostera;
• herbicides and pesticides inhibiting growth and causing decline of seagrass Zostera;
• chronic oil pollution and associated dispersant use affecting fauna, particularly epiphytic grazers.

Organic enrichment may be of less concern given that lagoonal sediments are naturally high in organic material. However, elevated organic inputs might be of concern in some cases because of low flushing rates in particular lagoons or parts of lagoons.

The characteristics of many lagoons (eg low flushing (or high retention times), fine sediments, stratification, species restricted to lagoons), suggest that once affected or impacted by inputs of contaminants, they may be slow to become clear of contaminants or to recover from associated impacts.

The few studies focussed on lagoons highlight that impacts as a result of changes in water quality can continue for some time or become self-perpetuating. Hodgkin and Birch (1982, 1986), for example, note complaints of excessive growth and accumulation of green algae from as early as the late 1960's. They describe a clear case of both recycling of nutrients from phyoplankton to benthic algae and a significant contribution from sediment stored phosphorus to growth of the main nuisance benthic algae. Furthermore, phytoplankton blooms had become progressively worse since the first major bloom without any corresponding increase in nutrient input. In terms of a management response, studies showed that even if the main source of inputs was removed (considered impractical anyway) the system would continue in a eutrophic state for 15 years (Hodgkin and Birch 1986). A management regime of harvesting excessive macroalgal growth was too late to address anything other than part of the symptoms and instead more radical and expensive management measures had to be considered (Hodgkin and Birch 1986).

In relation to nutrients, in particular, observations from estuarine systems are pertinent to lagoons. Scott et al (1999) note that a major concern with respect to the recovery of estuarine communities is that many of the responses to nutrient enrichment have been found to be self-perpetuating, i.e. once they begin to occur they create conditions which further create deterioration. Thus once threshold levels of impact are reached, relevant processes and associated detrimental impacts will continue without necessarily any further increase in nutrient inputs. Taylor et al (1998) provide an overview of these issues and discuss results from two Aeutrophic@ estuarine case studies. In freshwater systems analogous to saline lagoons, internal cycling of nutrients, particularly phosphorus, may mean that accumulation within bed sediments can contribute to nutrient status of a lagoon after inputs have been reduced (see Moss et al 1986) and thus delay recovery from any impact once it has occurred. Mainstone et al 2000 discuss this phenomenon in rivers.

In lagoons the low flushing time means that internal processes (biological and geochemical) will have a significant effect. Biological processes will include increased phytoplankton growth in response to nutrient inputs, subsequent death and retention of the resulting organic particulate matter and retention of nutrients arising from breakdown of this organic material. Particulate nitrogen in the form of detritus, for example, from benthic and floating macrophytes that respond opportunistically to nutrient enrichment, is also likely to contribute to the internal source of nutrients for subsequent uptake by algae. Other examples of processes that contribute to a self-perpetuating state include reduction in benthic-pelagic coupling, changes to benthic food webs and the uncoupling of nitrification/dentrification (see Scott et al (1999) for estuarine case studies and references).

As a consequence of the few case studies of lagoons, and observations in similar systems to lagoons in conjunction with the characteristics of lagoons, it is concluded that many, if not most, saline lagoons would have a low recovery potential from water quality impacts, and particularly from eutrophication impacts, within accepted management and planning time frames. This highlights the need to identify water quality impacts within lagoons as early as possible and suggests the need for a precautionary approach to interpreting and acting on information that may indicate an impact.

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