Environments and sensitivity

Introduction

Coastal waters differ in their physical conditions, ecology, and suitability for aquaculture. This page discusses the environmental factors that are relevant to aquaculture, and explains why some types of waters are more sensitive than others to the environmental 'pressures' resulting from aquaculture.

The word sensitivity is used here to mean the extent to which a given amount of aquaculture will result in an 'impact' on the ecosystem in which the aquaculture takes place. An ecosystem has a non-living part, its physico-chemical environment, and a living part, the biological community. We look at:

• the different kinds of environment in which aquaculture takes place, and at the physical conditions that influence sensitivity;
• the different kinds of biological community that are found in environments used for aquaculture, and at the 'resistance' of each type of community to aquacultural effects.

An ecosystem containing a 'sensitive' environment, or a biological community with low 'resistance', will have a lower capacity for safe assimilation of farm waste than a ecosystem of similar volume or extent and containing a more resistant biological community or environmental conditions that are less sensitive. As will be explored in greater detail below, environmental conditions that render an ecosystem less sensitive include greater dispersion at the farm scale and faster exchange at the water body scale. Ecological theory suggests that well-balanced biological communities - those that contain a variety of species and a mixture of lifeforms - are more capable of resisting 'pressure' (Tett et al., 2007).

In a report on 'the Environmental Impact of Aquaculture in Sensitive Areas' by Huntingdon et al. (2006) quote the European Environment Agency (EEA) definition of environmentally sensitive areas as:

Areas of a country where special measures may be given to protect the natural habitats which present a high level of vulnerability.

These habitats (and the organisms that inhabit them) may be more vulnerable because:

1. they are within an environment is sensitive (as defined here), or
2. the biological community lacks resistance to pressure, or
3. the habitat or organisms are rare, or economically valuable, and thus require a level of protection exceeding that which it is feasable to apply under normal circumstances and in the interests of 'sustainability' alone.

In the second and third cases in this list, EcoQOs are likely to be set more stringently than in typical cases. In the first case, a standard EcoQO will be exceeded for a smaller input of farm waste.

The Water Framework Directive requires special treatment for 'Protected Areas'. As listed in the directive's Annex IX, these include areas that are:

• protected under the Birds and Habitats directives;
• designated for the protection of economically significant aquatic species;
• recreational and bathing waters;
• 'sensitive' to enrichment with nutrients, including areas designated as vulnerable zones under the Nitrates Directive and areas designated as sensitive areas under the Urban Waste Water Treatment Directive.

Again, the 'assimilative capacity' of such waters for farm waste, may be deemed to be lower than the capacity of similar waters that are not designated as WFD Protected Areas.

There are differences between public environmental management regimes in different countries of Europe concerning the extent to which the natural sea-bed communities may be disturbed. Norway, for example, allows no visible disturbance. Scotland allows a small proportion of the sea-bed (that below farms) to suffer visible impact, on the grounds that the macrobenthic communities will quickly recover from local damage. Mediterreanean countries are only beginning to develope planning controls to prevent damage to seagrass beds. In some cases, historical rights to sites for bivalve aquculture has long ago resulted in replacement of the natural sea-bed communities by beds of mussels or oysters. In other cases, trawling by commercial fisheries has greatly damaged the natural communities. Nevertheless, it will be undesirable to site any sort of aquaculture so that it causes widespread or persistent damage to the natural flora and fora of the sea-bed: see Huntingdon et al. (2006).

No more will be written on this page concerning the aspects of sensitivity that involve special designation. The analysis will focus on general aspects of the ecology of sensitivity in relation to sustainable aquaculture.

An explanation of the terms 'pressure' and 'impact' will be found in the section dealing with DPSIR on the page about Management for Sustainability. This page also explains the term 'Ecological Quality Objective' (EcoQO) in the context of pressure-state-impact relationships.

Physical conditions in coastal water

The Water Framework Directive refers to two categories of waters that are relevant to European marine aquaculture:

• 'Transitional waters' are defined as "bodies of ... water in the vicinity of river mouths which are partly saline in character as a result of their proximity to coastal waters but which are substantially influenced by freshwater flow".
• 'Coastal water' is defined as "... water on the landward side of a line, every point of which is at a distance of one nautical mile on the seaward side from the nearest point of the baseline from which the breadth of territorial waters is measured, extending where appropriate up to the outer limit of transitional waters."

Annex II of the Directive lists the physical factors that must be, or can be, taken into account in identifying the type of water bodies falling into these two categories.

• Obligatory factors are salinity and tidal range;
• optional factors include: depth; current velocity; wave exposure; water residence time; mean water temperature and the range of temperature; mixing characteristics; turbidity; mean substratum composition; and shape.

Tett et al. (2007) considered the way in which these factors interact to control ecohydrodynamic conditions, especially:

• the availability of light for submarine photosynthesis, which helps to determine whether phytoplankton, seagrasses or seaweeds will be the most important producers of new organic matter for other creatures in the ecosystem;
• whether water bodies are mixed from sea surface to sea bed, or become layered, with warm water or reduced-salinity water floating over colder or salter water: the matter of stratification, which may change from winter through summer;
• the rate at which the contents of transitional water bodies and partly-enclosed coastal water bodies, exchange with the adjacent sea.

These ecohydrodynamic consequences influence both the type of biological community and the environmental control on sensitivity.

Temperature, salinity and oxygen

The temperature and salinity of sea-water have, of course, a direct effect on the success of aquaculture: fish grow faster, and eat more food, as the temperature increases; bivalve shellfish stop feeding when salinity falls too low. So far as environmental impact is concerned, the most important direct result of increasing temperature or increasing salinity is on the availability of dissolved oxygen. This determines the risk of low-oxygen conditions and consequent harm to biological communities (as well as to farmed animals). Thus, all other things being equal, warm and salty waters are more sensitive to fishfarm wastes than are cold and less salty waters.

When water contains as much oxygen as it can hold, it is said to be saturated. The diagram shows how saturation changes with water temperature and salinity, and relates it to some levels of oxygen at which the biota are affected by shortage of the gas. Concentrations are given in microMolars, abbreviated μM. One μM oxygen is 32 μgrams of oxygen per Litre of water (because a mole of O₂ is 32 grams and 1 Molar concentration is equal to 1 mole/Litre). Thus a typical seawater saturation oxygen concentration in Scottish coastal waters (at 10°C) of 300 μM, is 9.6 mg O₂ L^-1. Salinity is shown as lines for 0, 10, 20, 30 and 40, on the 'practical salinity scale'.

Layering of the sea

The layering of the sea is important to consider, because stratification acts a barrier to vertical mixing. Fish or shell-fish in a warm surface layer may gow faster because of higher temperatures and (in the case of bivalves) more phytoplankton. On the other hand, higher temperatures equates with a lower oxygen-holding capacity in water, as already mentioned. If the surface layer is naturally nutrient-depleted, there is greater risk of farm waste nutrient causing eutrophication. Particulate matter from farms sinks and hence may pass into a deeper layer, cut off from exchange with the air. Excessive decay can cause hypoxia in this layer.

Three factors tend to bring about water column layering:

• solar warming of the surface layers of the sea, because warm water is lighter than cold water;
• freshwater entering the superficial layers of the sea, because this water is lighter than salt water;
• colder or saltier (and therefore heavier) water penetrating beneath surface waters.

These factors are opposed by:

• mixing due to surface cooling, of the evaporation of freshwater, both of which make surface water heavier and so more prone to sinking ;
• stirring by wind, tidal currents, or other water flows.

These interactions, between stratifying processes and stirring processes, result in the following types of water column:

• perpetually mixed (where stirring rates exceed the rate at which heating or freshwater input induces stratification);
• Seasonal thermal layering, where there is a warm layer at the surface as a result of seasonal heating by the sun; such layering lasts for a few months in summer (at high latitudes) or for most of the year (in the Mediterranean Sea);
• persistent layering in some estuaries and fjords as a result of river discharge of fresh-water; in some cases, the deep water in fjords can stagnate for many months;
• alternately mixed and stratified, in places where the stirring and stratifying factors vary in intensity from day to day or week to week, and their competition results in a constantly changing outcome; such places are most often found where a varying discharge of freshwater interacts with variable winds and a strong difference between tidal currents during neap tides and spring tides.

Dispersion at the farm-scale

It has been said that 'the answer to pollution is dispersion and dilution', and this is indeed the case for many of the wastes produced by farmed fish or shellfish. The greater the rate of dispersion, the greater the biomass of finfish that can be raised for a given reduction in the concentration of oxygen or increase in the concentration of ammonia. The more widely waste food, faeces or pseudofaces are dispersed, and hence the smaller the impact per square metre of seabed. And finally, dispersion of old water equals replacement by new water, containing fresh phytoplankton as food for filter-feeding bivalves.

Two main processes disperse waste products away from a farm, or bring in new supplies of planktonic food:

• advection - the flow of water past a farm, driven by wind, tides, or an 'estuarine circulation'
• turbulence - the eddies that form when water flows quickly past an obstacle (including fish-cages) or close to the shore or sea-bed, or where two opposing currents meet.

An important question is this: how variable is the flow of water? In the following list, water flows are ranked in order of their ability to disperse, reliably, farm waste:

1. persistent strong currents, such as those due to persistent winds or to a large-scale pattern of circulation;
2. strong but periodic - mainly tidal - currents;
3. semi-persistent currents: e.g. wind-driven flows when the wind varies from day to day or season to season; an estuarine circulation driven by a seasonally-varying river discharge;
4. generally weak currents.

As has been stated elsewhere on these pages, strong local dispersion is the best single indicator of a good farm site.

Exchange (flushing) on the water body scale

In most cases, dispersion at the farm-scale simply mixes farm waste into the zone B scale water body, or mixes plankton to the farm from this body. An exception relates to sinking waste: in regions of low dispersion, this waste will reach the sea-bed near the farm, an stay there. If the waste includes nutrients, then it can cause eutrophication; if there is a release of toxic chemicals (e.g. used for killing sea-lice), then the chemicals may build up in the water body. Thus, an important matter is how quickly the water body exchanges with the adjacent sea. 

'Exchange rate' is the proportion of water-body contents that is each day exchanged with water from the open sea. An exchange rate given as 0.1 d^-1 implies that the instantaneous probability of a small packet of water leaving the water body, to be replaced by a similar packet from the open sea, adds up to 1 in 10 during a day. It is a good approximation to say that 10 percent is exchanged each day,

Substances contained in or floating in water will move with the water. The terms 'Dilution rate' or 'flushing rate' refer to the rate at which the water body's content of a pollutant, or similar, will decrease as a result of exchange. Dilution rate is exactly the same as exchange rate only in special circumstances (when the water body is completely mixed and when the pollutant is not influenced by other processes), but it is often roughly correct to equate the two variables.

Residence time is the median time that a molecule of water or a molecule of dissolved pollutant or a planktonic organism remains in a water body. In the special case of complete mixing etc, residence time is the exact inverse of exchange rate or dilution rate. If residence time is taken as an average, rather than a median, or if the water body is not uniform, then the relationship between residence time and exchange rate is only approximate. In practice, all these variables change with time, and their exact measurement is difficult, so the approximate inter-relationships are usually used as if exact.

Exchange between a water body and the surrounding or adjacent sea is the result of at least a dozen physical processes, including:

• large-scale (regional) circulations including persistent flow through straits
• wind-driven circulation
• tidal pumping, either:
  • the regular astronomical tide
  • irregular tides due to changes in atmospheric pressure
• density-driven circulations, including
  • estuarine circulation
  • intermediate exchange
  • deep-water replacement

Only some of these will be relevant for any particular water body

In general, faster exchange renders a water body to waste. All other things being equal, the assimilative capacity of a water body is proportional to the product of its volume and the exchange rate of that volume.

Tidal conditions

Strong tidal flows provide good dispersion around farm sites, and in some cases increase the rate of exchange between a water body and the adjacent sea. The Water Framework Directive categorizes tidal conditions as follows:

• mean tidal range less than 2 m: microtidal;
• mean tidal range betwee 2 and 4 m: mesotidal;
• mean tidal range more than 4 m: macrotidal.

Most coastal waters on the Atlantic side of the Skagerrak and the Strait of Gibraltar are mesotidal or macrotidal; the Baltic Sea, the Mediterranean Sea, and a few regions elswhere are microtidal. Because of the importance of tidal conditions for aquaculture, it is worth considering what is meant by 'the tide' and why it differs so markedly between different European seas.

'The tide' is commonly understood in terms of regular changes in sea-level, and the currents required to bring about these changes in sea-level by moving water from one part of a coastal sea to another. There are two causes of these water movements and the changes in sea-surface height.

• the atmospheric tide is caused by differences in atmospheric pressure between two parts of the sea;
• the astronomical tide is caused by the gravitational attractions of the moon and the sun, which act differently on the solid Earth and the contents of the oceans and other large water bodies.

The astronomical tide originates in waves that rotate around the main ocean basins, which resonate to the solar-lunar pull that occurs when these bodies are either overhead or underneath the planet. The waves move at thousands of kilometres per hour, but are the result of water molecules orbitting a few tens of centimetres during tidal cycle. As oceanic high tide rotates past the edge of a given continental shelf, however, the wave propagates into the shelf sea, and can send water rushing into, or out of coastal regions. This is the case along the open Atlantic coast of north-western Europe; in shelf sea basins such as the North Sea or the Irish Sea, local resonance reinforces the tide. These are the mesotidal and macrotidal regions. Such resonance may be thought of as like water in a bath, which can be caused to slosh back and forth if disturbed at the correct rate. Note that, in the middle of such a bath, water level does not change, but water flows are strong; at the ends of the bath, flows are weaker but height changes are greater. This analogy applied to some parts of the mesotidal regions. Around the southern coast of Norway, or in soe western Scottish inshore waters, tidal range is small, but tidal currents are strong.

Conditions are very different in the Baltic Sea and the Mediterranean Sea. The oceanic tidal wave is damped by the Skagerrak-Kattegat and the Strait of Gibraltar, and the size and shape of the two Seas is such that they do not themselves resonate to solar or lunar pulls. Thus both the range in sea-level, and the maximum currents, due to the astronomical tide, are small here: typically much smaller than the 2 m range given as the upper limit of microtidal conditions.

All European seas experience atmospheric tides, which can in some cases add a metre or more to the range of the astronomical tide - as exemplified in 'storm surges' in the southern North Sea. Flooding in the lagoon of Venice is mainly the result of such an atmospheric tide, with the resulting wave being funelled and, hence, intensified, as it passes up the Adriatic Sea. Elsewhere in the Mediterranean the atmospheric tide is small. In most places, significant variations in sea level are infrequent, and the resulting currents make little contribution to farm-scale dispersion or water-body exchange. There are some exceptions: the Lagoon of Thau, in southern France, is flushed almost entirely by this tide, which pumps water into the Lagoon when sea-level rises outside; water runs out when external sea-level falls.

Water transparency and the biological community

Water transparency and water depth control the amount of light reaching the sea-bed, and this interacts with sediment type to control the kind of organic community found here. Sediment type, and the tendency to accumulate fine or light particles, is itself a function of energy levels in the water: waters with weak currents, little turbulance, and sheltered from the wind and waves, are called 'low-energy'. The following table sets out the main types of seabed communities, and the risks from aquaculture, likely in most European (cold or warm temperate) waters.

The euphotic zone is that within which there is sufficient light for the growth of photosynthetic organisms such as grasses or algae. It extends to depths of up to 50 metres depth in naturally clear waters, such as those in parts of the Mediterreanean Sea. It may be only a few metres deep in some naturally turbid coastal waters of NW Europe.

The diagram below shows the suitability, of four distinct examples of marine water-columns, for the growth of different types of photosynthetic organism, and illustrates some of the distinctions made in the table.

Types of coastal environment used for aquaculture

Here we present two tables that list some of the main types of European waters in which aquaculture is carried out, and which have been studied during ECASA. The water types are broadly categorized into:

• Regions of Restricted Exchange (RREs)
• open coastal waters

which are the subject of the two tables that follow. Annex XI of the Water Framework Directive places European transitional and coastal waters in 6 ecoregions:

1. Atlantic Ocean
2. Norwegian Sea
3. Barents Sea
4. North Sea
5. Baltic Sea
6. Mediterranean Sea

ECASA did not study regions 3 or 5, nor the microtidal part (in the Skagerak and Kattegat) of region 4. In what follows, mesotidal and macrotidal waters are to be found in the Atlantic Ocean, (western) North Sea and Norwegian Sea ecoregions and microtidal waters in the Mediterranean.

Regions of Restricted Exchange

RREs are defined as: water bodies that are enclosed on three sides, so that exchange with open water is strongly controlled by conditions at the entrance. The entrance width is less than the water body's length. Within the table they are categorized on the basis of defining physical features (especially, shape and consequences for exchange) and the tidal conditions.

Open Waters

This second category is for sites in water bodies that exchange with the open sea across at least two sides.

Summary

Some farm sites and some water bodies are more sensitive to the environmental effects of aquaculture than others. This page has dealt with ecohydrodynamic factors that:

• influence the environment's capacity to remove wastes from, or supply food to, farmed animals;
• influence 'natural' ecological conditions at the site or in the water body.

In relation to the theory of management for sustainability,

• the first category relates to the slope of the pressure-state/impact relationship: more sensitive environments are those in which a given increase in pressure results in a greater change of state and greater probability of impact;
• the second category helps fix the level at which tolerance thresholds are set;

Models

The models studied by ECASA fall into two broad categories in relation to environmental factors:

• zone A scale models that are specific for type of cultivated animal, but which can be applied in almost all environmental types:
  • BREAMOD, etc;
  • DEB, DDP;
  • DEPOMOD, etc;
  • MOM;
  • ShellSIM;
• zone B scale models that are designed for particular sorts of environment:
  • Narrow and deep RREs, including fjords, rias and inlets: FjordEnv, dCSTT, LESV;
  • Shallow or mixed RRES: Longlines;
  • Any RRE that can be described by one box (CSTT) or multiple boxes (EcoWin);
  • Open coastal waters: KK3D, TRIMODENA;

Links to be made to each model page.

See also the page dealing with hydrodynamic models.

References

• Huntington, T.C., Roberts, H., Cousins, N., Pitta, V., Marchesi, N., Sanmamed, A., Hunter-Rowe, T., Fernandes, T.F., Tett, P., McCue, J., & Brockie, N. (2006). Some Aspects of the Environmental Impact of Aquaculture in Sensitive Areas. Report to the DG Fish and Maritime Affairs of the European Commission., Rep. No. 221-EC/R/02/B. Poseidon Aquatic Resource Management Ltd, Windrush, Warborne Lane, Portmore, Lymington, Hampshire SO41 5RJ, UK.
• Tett, P., Gowen, R., Mills, D., Fernandes, T., Gilpin, L., Huxham, M., Kennington, K., Read, P., Service, M., Wilkinson, M., & Malcolm, S. (2007) Defining and detecting Undesirable Disturbance in the context of Eutrophication. Marine Pollution Bulletin, 53, 282-297.