Nutrient enrichment and eutrophication
Several forms of anthropogenic water pollution can cause loss of, or
damage to Zostera beds. This can occur rapidly, for example where plants are killed
by water-borne toxins or smothered by oil, or over a longer time-scale, as when nutrient
input causes eutrophication, with associated increases in turbidity and proliferation of
There has been relatively little research on the effects of chemicals,
other than oil or dispersants, on the growth and survival of seagrasses, but heavy metals,
antifoulants and herbicides are all thought to have the potential to cause harmful
A review by Williams et al. (1994) summarized current knowledge on
heavy metal uptake and toxicity in saltmarsh plants, including Z. marina. They
suggested that since Z. marina readily takes up heavy metals, mainly through the
leaves, this species could be used as an indicator species for heavy metal levels in the
surrounding water and sediments. However, they found that heavy metals had not caused any
observable damage to Zostera plants in the field and concluded that the
concentration of heavy metals in most estuaries was not sufficiently high to cause ill
However, Brackup et al. (1985) investigated the effects of a number of
pollutants on the nitrogenase activity of Z. marina roots. They found that several
heavy metals (mercury, nickel and lead) along with a number of organic substances
(naphthalene, pentachlorophenol, Aldicarb and Kepone) reduced nitrogen fixation, which may
affect Zostera viability.
Zostera marina is known to accumulate tributyltin (TBT), found in
antifouling paints (Francois et al., 1989), but Williams et al. (1994) found that TBT had
not caused any observable damage to Zostera plants in the field.
Research on the effects of the triazine herbicide Irgarol, used in
antifouling paints, on Z. marina has shown that this herbicide is present in the
roots and shoots. Triazine herbicides are specific inhibitors of photosynthesis and
sublethal effects have been detected (P. Donkin, pers. comm.).
Terrestrial herbicides entering the coastal zone via streams and rivers
may damage eelgrass. The herbicide Atrazine has been implicated in declines of Z.
marina in Chesapeake Bay (Hershner et al., 1983). Delistraty & Hershner
(1984) studied the effects of Atrazine on Z. marina and found exposure to 100 ppb
over 21 days resulted in growth inhibition and 50% mortality.
In the modern coastal environment, marine biotopes may be exposed to
oil pollution from a number of different sources. Continuous, long-term exposure to small
concentrations of hydrocarbons may result from proximity to coastal oil refineries,
industrial installations or harbours. Conversely, massive, effectively instantaneous
exposure will occur in association with major pollution incidents such as oil spills or
tanker wrecks. These contrasting degrees of exposure may have quite different consequences
for eelgrass beds.
Long-term exposure to refinery effluent
Sensitivity to chronic exposure to oil refinery effluent may not be
very high. K. Hiscock (1987) reported that there were no long term effects attributable to
the presence of an oil refinery on Zostera in Littlewick Bay, Milford Haven, Wales,
but suggested that this may be due to the effluent not penetrating into this area.
Cambridge et al. (1986) and Shepherd et al. (1989) reported that Posidonia sinuosa
in Western Australia was rather insensitive to oil refinery effluent based field
observations and experiments in aquaria. However, they did attribute some local,
small-scale, declines to this cause.
Major oil spills
In the event of an oil spillage, the likely impact depends upon a
number of factors including the type of oil, the degree of weathering and the nature of
the habitat. A number of studies have suggested that, in general, it is the associated
faunal communities that are more sensitive to oil pollution than the Zostera plants
themselves (Jacobs 1980, Zieman et al., 1984, Fonseca, 1992). Rocky shore gastropods such
as limpets, have been found to be very sensitive to dispersants or oil dispersant
mixtures, and it is possible that gastropod grazers on Zostera epiphytes may be
equally sensitive, which could potentially result in epiphyte overgrowth.
As Z. noltii occurs highest up the shore, this species is likely
to be the most vulnerable to covering by oil while Z. angustifolia and Z. marina
may be partially protected by seawater from direct contact with the oil. Since Zostera generally
occurs in sheltered, low-energy sites, natural weathering of oil will be slow.
Jacobs (1980) reported little damage to Z. marina in Roscoff
after the Amoco Cadiz spill, other than a blackening of the leaves for 1 - 2 weeks. He
observed that the growth, production and reproduction of the plants were not affected,
despite the leaves being covered for a period of six hours. The fauna of these eelgrass
beds was slower to recover than the eelgrass itself.
Treatment of oil spills
Major oil spills are often treated with chemical dispersants to
encourage break-up of the oil layer. In some cases, these dispersants have been found to
be cause greater damage to biological communities than the oil itself.
Holden & Bakers (1980) 11- 16 month studies in Milford Haven
found that a single application of either (i) Forties crude oil, (ii) BP 1100 WD
dispersant, (iii) oil followed by dispersant or (iv) pre-mixed oil and dispersant, could
reduce growth of intertidal Zostera. Howard (1986) repeated these earlier
experiments and found that that three of the treatments; Nigerian crude oil, Dispolen 34
dispersant and oil followed by dispersant, arrested growth but appeared to cause little
change in the Zostera cover. In comparison, the plots treated with the pre-mixed
oil and dispersant showed rapid death of the leaves and a significant decline in cover one
week after application. By the end of the eight week experimental period, cover had been
reduced from 55% to 15%.
Howard (1986) also found that pre-mixing the oil and dispersant
promoted penetration of oil into the sediment, resulting in a hydrocarbon concentration of
more than 7000 ppm 24 hours after application, compared with 1000 ppm in the other
treatment plots. The high oil concentrations were not retained within the sediment, and
within a week of application, all oiled plots had hydrocarbon concentrations of 500 - 1000
ppm. The main findings of the Milford Haven research are generally similar to the
conclusions reached by other researchers, namely that pre-mixed oil and dispersants have
the greatest potential for killing seagrasses, whereas contact with oil alone may reduce
or halt growth.
Howard et al. (1989) concluded that if oil cannot be prevented from
covering eelgrass beds, then dispersant treatments must be avoided in order to minimize
the risk of a partly dispersed oil mixture affecting the eelgrass. It was advised that oil
coverage on eelgrass beds should be left untreated, and the oil layer allowed to disperse
by tidal action.
Nutrient enrichment and eutrophication
Eutrophication (excessive proliferation of planktonic or benthic algae)
can be caused by increased nutrient inputs, originating from sewage, agricultural runoff
or aquaculture. In some cases, local increases in nutrient levels appear to have
favourable consequences for eelgrass beds. This is likely to occur in situations where Zostera
growth is limited by available nitrate (Fonseca et al., 1987, Kenworthy & Fonseca,
1992, Fonseca et al., 1992). Tubbs & Tubbs (1983) reported that rapid increases in the
extent of Zostera beds paralleled increased volumes of treated and untreated sewage
entering three areas of the Solent. However, it was concluded that despite the spatial and
temporal associations, there was no direct evidence of a causal relationship.
Eutrophication is more often cited as a major cause of the decline, or
the lack of recovery of, Zostera beds (Borum, 1985; Wetzel & Neckles, 1986; den
Hartog & Polderman, 1975; Orth et al 1983; Shepherd et al., 1989; Kikuchi,
1974). A variety of different harmful effects have been identified. These are not mutually
exclusive, and several or all of them may apply in any given situation.
High nitrate concentrations and metabolic imbalance
High nitrate concentrations have been implicated in the decline of
mature Z. marina (Burkholder et al., 1992). The meristems of the plants were found
to deteriorate and it was suggested that high internal nitrogen concentrations caused a
metabolic imbalance. Burkholder et al. (1992) found that nitrate enrichment could cause
death or decline in seagrasses, including Z. marina in poorly-flushed areas. They
found that Z. marina was more sensitive than Halodule wrightii and Ruppia
maritima and that the effect was exacerbated by heavy epiphyte growth. In the Dutch
Wadden Sea, declines in Zostera since 1965 may have been associated with increased
nutrient levels (Den Hartog & Polderman, 1975; Polderman & den Hartog, 1975).
Kikuchi (1974) made a similar suggestion about the decline of Zostera beds in
Increased growth of epiphytic algae
Many studies have correlated seagrass loss with increased growth of
epiphytic, blanketing or floating algae, often as a result of eutrophication (e.g. Borum,
1985; Burkholder et al, 1992; Orth et al., 1983; Shepherd et al., 1989; Wetzel
& Neckles, 1986).
Increased growth of epiphytic algae can result in increased numbers of
epiphyte grazers. However, populations of grazers may not be able to respond rapidly
enough to utilize and control the epiphytes and the Zostera plants may still become
Nutrient enrichment can also encourage rapid growth of blanketing
algae. Some opportunistic species such as Enteromorpha sp., Ectocarpus
confervoides and Ceramium rubrum may cause severe shading of Zostera (Den
Hartog, 1987). Den Hartog (1994) reported that at Langstone Harbour, the growth of a dense
blanket of Enteromorpha radiata in 1991 resulted in the loss of 10 ha of Z.
marina and Z. noltii, and that by the summer of 1992, Zostera was
Phytoplankton blooms, resulting from nutrient enrichment, can increase
turbidity and have been shown to reduce the biomass production and the depths to which Z.
marina can grow (Dennison, 1987). Increased turbidity caused by phytoplankton blooms
has also been implicated in the loss of seagrass beds in Australia (Shepherd et al.,
Increased vulnerability to wasting disease
Buchsbaum et al. (1990) found that the levels of phenolic compounds
were lowered under conditions of nutrient enrichment, possibly due to a reduction in
available carbon within the plant. Phenolic compounds play an important role in providing Zostera
with defence against infection, including wasting disease. Burkholder et al. (1992) found
that plants from enriched mesocosms succumbed to infection by Labyrinthula macrocystis,
while plants in the control mesocosm remained healthy.