feeding. The activities can lead to mortalities through the control by shooting (which requires an exemption to bird protection Acts in certain countries) and there may be accid- ental or deliberate trapping of birds (through the use of anti-predator nets etc.). These adverse effects may be minimized by site con- siderations, gear improvement, and the use of scaring devices or sacrificial food, and the adoption of a policy on shooting.
Aquaculture requires the addition of chem- icals and these may be given as enteric treatments (by mouth, e.g., with the fishfood) or by immersion treatments. These include food additives such as vitamins, mineral mixes, and pigments, the latter are required in the case of salmonids in order to produce pink flesh, which is acquired in the wild by eating a crustacean diet not used in farming.
Anaesthetics and narcotizing agents are required to minimize stress of the fish during handling and vaccines and other therapeutants are administered for health. Disinfectants and antibiotics will be used to minimize microbial effects and, given the increased possibility of external parasite transfer when large numbers of fishes are confined, pesticides (such as Nuvan/Aquaguard (dichlorvos based) and ivermectin) are used to combat infestation of fish lice. Finally, antifouling agents/
treatments (such as Cu, TBT, bitumen, and slip- paints) may be used to control macrofouling of the gear.
Aquaculture thus has the potential for impacts at different biological levels. Manage- ment strategies may be employed to reduce or remove the effects of aquaculture. The deve- loper may collect the excess solids by funnel or suction system although decomposition, a release of nutrients, and poor water quality can still occur. Pumping or bed trawling to aid breakdown could disperse the wastes. Good siting principles will minimize the effects or the cages may be moved at intervals, with those intervals being judged by the degree of biotur- bation under the cages, hence using the assim- ilative capacity of the area. Feeding techniques
using low-wastage feed and hand-feeding, coupled with the type of food (slow sinking, and with a balance controlled to reduce NH3 input) will reduce the effects. The use of good practice such as the removal and destruction of mortalities, rather than into the water column, and the use of biological control for pests (e.g. lice control using goldsinny wrasse) will minimize biological effects. In addition, fish- farmers and environmental protection agencies determine the potential effects using numerical modeling (based on volume of stock, water flow, depth), coupled with self-monitoring for the health of sediments and benthos. Sampling and/or photography then achieve the assess- ment. Finally, the use of polyculture, as a mix- ture of fin- and shell-fish farming, or the use of ranching as compared to cages can be a wiser use of the environment.
6.8 Industrial contamination
Waste from industrial sources enters estuaries from industries located on the banks of the estuary as well as that from inland sites may be discharged into rivers or public sewers, which subsequently arrive in estuaries. The waste from industrial sources may be treated before discharge, so that the effluent arriving in the estuary has a considerably reduced concentration of waste materials, or the waste may be discharged in an untreated form. All industries produce some amount of waste, ranging from large volumes of water, which have been used for cooling purposes, through to chemical waste products which may be extremely toxic even in small quantities.
6.8.1 Petrochemical discharges and inputs
one-third of all the oil discharged into the world’s oceans. Thus estuaries do indeed receive a disproportionately large burden.
Whereas oil spillages and tanker accidents receive a large amount of press coverage, by far the greatest proportion of oil enters the estuaries and coastal seas from diffuse sources such as urban and river runoff or domestic or industrial waste. The effects of the oil industry on estuaries may be divided into (1) the impact of gross spillages, due to shipping accidents or human error at a loading terminal, (2) the impact of effluents produced by refinery and petrochemical industries, and (3) the effects of oil extraction.
Oil spillages
Oil spillages from harbors or collisions have many and varied potential effects, which are the result of the nature of the oil spilled, the characteristics of the estuary and the climatic conditions. Some of the different fractions may be toxic while others are relatively inert and all the fractions will be subject to physical, chemical, and biological degradation. When oil is spilt from a harbor or a collision it reaches water as a separate phase, which is largely immiscible with the water, and gen- erally forms a surface slick (Fig. 6.15). The slick will spread and be subject to physical and chemical breakdown through photo- oxidation, evaporation and, during high winds, aerosol formation. Hence the import- ance of the weather conditions affecting the fate and effects of the spilled oil. Many of the lighter and most toxic elements can be lost by weathering, but unfortunately, in the confined space of an estuary there may not be time for this to occur before the spilt oil is deposited on the shore.
The surface slick and the sinking oil will be transported to areas of the estuary depend- ing on the wind and tidal conditions and the nature of the estuary. For example, a spillage at the mouth of the estuary will impact on local rocky shores and sandy beaches whereas that pushed up the estuary
could affect mudflats, salt marshes, and reedbeds. Once on the sandy beaches, the oil can be pushed into the coarser sediment by tidal pumping and once in the sediment, espe- cially if there is no oxygen penetration then aerobic degradation will be reduced. Hence the oil can remain for many months. On the shore the oil acts mechanically, smothering animals in burrows or on rocks, and excluding the light from plants. Given that mudflats have larger infauna and predator populations than sandy beaches then oil deposited on the for- mer has a greater smothering effect and affects predator–prey relationships. Oil deposited on higher energy sandy beach surfaces and on rocky shores will be subjected to greater phys- ical degradation and thus may breakdown more easily. In contrast, oil deposited in low energy estuarine environments such as mudflats, seagrass beds, salt marshes, and reedbeds will persist for longer and is also more difficult to remove by mechanical meth- ods. It will infiltrate creeks and between the plants and thus be protected from mechanical breakdown. On a salt marsh the soiled parts of the plant die, but if it can survive this, the plant can regrow.
The oil will also be mixed with the water column forming first water-in-oil emulsions and then, with increasing mixing, oil-in-water emulsions (often called chocolate mousse due to the color and consistency). Zooplankton will ingest the oil droplets and produce fecal pellets, which can aid breakdown by bacterial biodegradation on the large surfaces. These droplets and pellets may then settle out of the water column. The water-soluble or more volatile compounds are most toxic, and spillages in American estuaries of fuel-oil containing 45% low aromatic oils have devas- tated commercial shell fisheries. The heavier compounds within the oil, which remain after evaporation, will tend to sink, where they undergo microbial decomposition by many microorganisms, which are capable of degrad- ing petroleum hydrocarbons as well as naturally occurring biogenic hydrocarbons.
Atmosphere
Shore Shore
Oil (surface spill)
Stranded oil
Aerosol formation
Oxidation and evaporation
Precipitation and dry fall-out
Oil slick Mousse lumps Tar balls
(oxidation) Stranded
beach tar
Solution (water soluble fraction) Absorption
and Adsorption
Convection Dispersion Up-welling
Micro-particulate tar Solution, etc.
Absorption and Adsorption, etc.
Dispersion, etc.
Oil (subsurface spill)
Emulsification Chemical degradation
Dispersal CO2 In suspension Biodegradation
Food chain
Detritus Fecal
pellets (zooplankton)
Sea bottom
Sediment Sediment Resuspension
Reworking benthos
Sedimenting residues
Biodegradation and fauna
Long term burial
Figure 6.15 The fate of oil spilled or discharged into surface waters (modified from CONCAWE 1981, www.concawe.org).
The most resistant oil fractions, the heavier and less volatile components, however, remain as tar-balls for many weeks if not months.
Given the serious consequences of oil pene- trating the more delicate habitats in estuaries, especially those where oil is difficult to remove once it settles, then the most common advice on how to deal with oil spills in estuar- ies is to contain the oil by a boom if possible, and then to use mechanical removal with pumps for the trapped oil. If it comes ashore on sandy beaches then mechanical removal with shovels or excavators is the preferred mode of treatment, rather than emulsifiers if at all possible. The earlier emulsifiers, or detergents, used in clean-up campaigns to disperse oil were often more toxic to estu- arine life than the original oil. Newer emulsi- fiers are less toxic. Oil washed off rocky shores with hot and/or freshwater can cause greater community effects than the oil if left alone. Oil settling on to mudflats, salt marshes, sea grass beds and reedbeds in estuaries cannot be removed without con- siderable damage to those habitats and so if these areas are oiled then the decision is very often to leave the oil to natural clean-up.
Consequently, an estuarine oil-spill contin- gency plan may involve recovering the oil from sandy beaches rather than other habitats, especially as the beaches may have good access and will support heavy machinery.
Figure 6.16 shows the recommended decision processes involved in dealing with an oil spill within an estuary.
Petrochemical discharges
Less dramatic, but potentially more dangerous to estuaries than oil-spills, are the insid- ious effects of the continual discharge of industrial effluents from petrochemical com- plexes. The effluent from such industries is mostly hot freshwater, which may contain certain amounts of chemicals, including hydrocarbons (oil) from refineries, or chemical waste products such as phenols or ammonia
from chemical works. A refinery effluent may contain only 10–20 ppm of oil, which is dif- ficult to remove and seen perhaps only as cloudy water with an oily surface sheen, but as a flow of 7.58 cumecs this produces more 6.55 tonnes of oil per day. The impact of efflu- ent from petrochemical complexes in the Medway and Forth estuaries was examined before major clean-up operations. In the vicinity of the outfall an abiotic zone was found, with no life at all. Beyond this was a “grossly polluted zone” in which only Oligochaetes were found, in small numbers.
Then followed a “polluted zone” with abun- dant Oligochaetes and Manayunkia (a small polychaete), and occasional specimens of other species such as Hydrobia. Finally came the “largely unpolluted zone” at over 1-km from the effluent, with Macoma, Cerastoderma, Nereis, Nephtys, and Hydrobiaoften abundant, and fewer Oligochaetes. It may be seen that the zonation of species and their abundance is remarkably similar to that noted above for the effects of organic enrichment. Since oil is a biological product derived geochemically from organic material, it is perhaps not so sur- prising that the effects of oil on estuarine ecosystems are rather similar to the effects of other excess organic materials. Throughout the world oil refineries have made strenuous efforts to reduce their waste effluent, and modern oil refineries have considerably less impact on the estuarine ecosystem than their older counterparts. When an oil refinery dis- charge is reduced, or terminated, then the resilient estuarine fauna can generally recolo- nize the area rapidly.
Oil and gas extraction
Oil is extracted from several estuarine loca- tions, most notably the Nigerian oilfields are located within Nigeria’s estuarine delta region, and blow-outs from the wells have occasionally led to environmental problems.
The shallow areas of eastern England and the Dutch coast border on the southern North Sea gas fields and the large estuarine area
Morecambe Bay in NW England also sits on a gas field and thus these areas support gas rigs as well as seabed collecting systems. Gas and oil come ashore by pipeline into several estuaries, and so formation water and produc- tion water has to be treated and disposed.
Formation water is the fossilized water
trapped with the oil and gas, which may contain high levels of minerals and be at a high salinity whereas production water is that pushed down into ageing wells to increase production. Each of these will contain oily residues and thus require treatment before disposal into the estuary.
Oil spilled
Moving offshore Continue observing
Moving on-shore Observe or predict
No
No
No
No No, or No
partially
Can oil type and condition be chemically dispersed?
Is a dispersion operation possible?(a)
Clean up on-shore
Will adverse impacts associated with chemical dispersion be less than those resulting without chemical dispersion?(b)
Chemical dispersion acceptable (a) Availability materials and
logistic support (b) Input from prior comingency planning includes environmental considerations
Clean up on-shore Continue
actions Yes
Yes
Yes
Yes Yes
Implement Are control/recovery,
actions adequate?
Is physical control and recovery feasible?
Yes
Determine oil characteristics;
consider location, sea state, hydrography, meteorology Leave alone
Determine if resources, amenity, etc. at risk
Figure 6.16 Decision “tree” for the response to an oil spill in an estuary, considering in particular whether a chemical dispersant is acceptable. (From Concawe, 1981.)