Entry to the marine environment
Organic carbon or organic matter enters the marine
environment from allochthonous (external) and autochthonous
Natural allochthonous sources include river-borne
phytoplankton and organic detritus, and marginal
vegetation. These natural sources are supplemented
considerably by anthropogenic point sources including
sewage effluent, some industrial effluents and cage
fish farm installations. The activities of Man in
river catchments can increase the export of organic
detritus to rivers, estuaries and coastal waters.
Natural autochthonous sources include phytoplankton,
marginal submerged vegetation, faunal faeces and
pseudofaeces. The increase in loads of nutrients
(nitrogen and phosphorus) to the marine environment
as a result of Man's
activities stimulates the production of autochthonous
organic matter, principally in the form of phytoplankton
and submerged marginal vegetation (macroalgae).
For example, organic matter is derived from decaying
seaweed (seasonal scouring of macroalgal, notably
kelp communities, can result in large amounts of
decaying seaweed being washed onto the shore - in
some years more than 40,000 tonnes per year of stranded
seaweed is removed from the beach at St Helier,
Jersey (Parr et al 1996)). Deposits of organic
matter in the marine environment can be redistributed
by storm events.
Recorded levels in the marine
Organic matter occurs in natural waters in dissolved
(dissolved organic matter DOM) and particulate (particulate
organic matter POM) forms. DOM is the organic matter
passing through a 0.5 mm filter and POM is that
retained by the filter.
The concentration of organic matter does not appear
to be routinely measured in UK tidal waters, but
the concentration is likely to vary enormously on
both a temporal and spatial basis, depending on
river flow, resuspension of sediment during windy/stormy
weather. However, total suspended solids and ash-free
suspended solids are monitored (subtracting the
latter from former provides a measure of the particulate
organic matter (POM) of water samples). While inorganic
material comprises the majority of total suspended
solids in English and Welsh coastal waters, along
the Wash and North Norfolk coast, organic material
is responsible for a greater proportion of total
suspended solids (Parr et al 1998).
Total organic carbon (TOC) is frequently measured
in sediments, particularly in monitoring programmes
of sediment quality in relation to the disposal
of organic wastes.
Fate and behaviour in the marine
The fate and behaviour of organic matter in the
marine environment is complex and is usefully reviewed
for estuaries in Kennish (1986).
Dissolved organic matter (DOM) comprises substances
from biological origins (e.g. polypeptides and polysaccharides)
and from geological origins (e.g. humic substances).
Much of the biological DOM is metabolised by heterotrophic
bacteria, while most of the geological DOM is resistant
to microbial breakdown. Some other animals can use
biological DOM but are largely outcompeted by bacteria
and the proportion they use is not nutritionally
important for them.
Particulate organic matter (POM) is also metabolised
by heterotrophic bacteria but is also used directly
by a wide range of estuarine invertebrates both
in the water column and in the sediment. POM can
be suspended in the water column or be deposited
onto sediments, depending on the size and density
of particle and the current velocity.
Organic carbon is readily assimilated into the
tissues of marine organisms, can be transformed
from POM to DOM and is lost to the atmosphere as
carbon dioxide by respiration.
The addition of large amounts of organic matter
from anthropogenic sources can exceed the capacity
of parts of the marine environment to process it,
resulting in accumulation usually in the sediments.
Effects on the marine environment
The effects of non-toxic substances, such as organic
carbon, on the marine environment can be sub-divided
into direct effects (those organisms directly affected
by changes in the concentrations of organic carbon)
and secondary effects (those arising in the ecosystem
as a result of the changes in the organisms directly
The addition of organic matter (as DOM or POM)
to the marine environment has the direct effect
of stimulating heterotrophic bacterial production
in the water column and in the sediments.
The addition of POM, in particular, stimulates
the production of invertebrate detritivores in the
water column. The response of benthic invertebrate
communities to increasing inputs of organic material
has been characterised by Pearson and Rosenberg
(1978). There are two distinct phases in the response
often referred to as organic enrichment and organic
Organic enrichment encourages the productivity
of suspension and deposit feeding detritivores and
allows other species to colonise the affected area
to take advantage of the enhanced food supply. The
benthic invertebrate community response is characterised
by increasing numbers of species, total number of
individuals and total biomass. This type of impact
is typical of many modern sewage effluent discharges
in the marine environment (e.g. O'Reilly
et al 1998).
Organic pollution occurs when the rate of input
of organic matter exceeds the capacity of the environment
to process it. Commonly, there is an accumulation
of organic matter on the sediment surface that smothers
organisms, depletes the oxygen concentrations in
the sediment and sometimes the overlying water which
in turn changes the sediment geochemistry and increases
the exposure of organisms to toxic substances associated
with organic matter. The benthic invertebrate community
response is characterised by decreasing numbers
of species, total number of individuals and total
biomass and dominance by a few pollution tolerant
annelids. This type of impact is not common other
than in localised areas in the estuaries and coastal
waters of the UK but has recently been observed
in relation to cage fish farm installations.
Eleftheriou et al (1982) showed that an
organic loading rate of 2.1 g C.m-2.d-1
in a sea loch resulted in an enriched sediment fauna,
while a loading rate of 4.1 g C.m-2.d-1
on marine mesocosms degraded the sediment community.
Mesocosm studies at the University of Rhodes (Kelly
and Nixon 1984, Frithsen et al 1987, Oviatt
et al 1987 and Maughan and Oviatt 1993) showed
that, while organic loading rates of 0.1 g C.m-2.d-1
had little effect on benthic community status, loading
rates of 0.1-1.0 g C.m-2.d-1
produced an enriched community and rates in excess
of 1.5 g C.m-2.d-1 degraded
the benthos community.
Edwards and Jack (1994) suggested that the loading
rates at which detrimental effects were reported
varied so much between the mesocosm and sea loch
studies because all of the organic matter applied
in the mesocosms was deposited in the experimental
system while in the sea loch study, a large proportion
of the organic matter applied was probably lost
from the immediate area being studied.
The model BenOss predicts the pattern of sedimentation
and biological impact of organic matter around marine
outfalls, relying on sedimentation coefficients
for a range of particle sizes while taking account
of dispersion by currents and turbulence. The model
employs critical resuspension speed of 9.0 cm.s-1
and a critical deposition speed of 4.5 cm.s-1
(Cromey et al 1996). The model also predicts
the effects of excess organic carbon from outfalls
on the benthic faunal population.
Increasing amounts of both DOM and POM can contribute
towards increased turbidity in the water column.
Stimulation of microbial degradation in the water
column and sediments results in an increased oxygen
demand and decrease in available oxygen for other
organisms. Microbial degradation of organic matter
also results in the mineralisation of nutrients
(N and P) providing additional sources of these
nutrients for phytoplankton and macroalgae. The
effects of eutrophication and organic enrichment
are difficult, if not impossible, to distinguish.
However, the distinction is only important when
considering control mechanisms on the supply of
nutrients (N and P) and organic matter.
The effects of increased oxygen demand in water
and sediments are considered in more detail in the
section on dissolved oxygen. In the water column,
reduced oxygen concentrations can result in dissolved
oxygen sags (DO sags) in estuaries which can act
as a barrier to migratory fish and damage estuarine
fish communities. The combined effects of organic
pollution by sewage, including the input of organic
carbon and consequent depletion of dissolved oxygen
in the water column, resulted in the eradication
of the fish populations in the Thames estuary. Judicious
management of water quality, including the artificial
aeration of the water column, has allowed the fish
populations to recover almost completely. Such a
dramatic effect on the fish community must have
had an equally dramatic effect on piscivorous birds
using the estuary.
Organic enrichment of sediments in the marine environment
can stimulate the production of benthic invertebrate
communities which, while different in composition
from pre-enrichment conditions, can provide significant
food supplies for fish and birds. It has been suggested
that some of the bird populations at European marine
sites in estuaries are sustained by benthic invertebrate
production resulting from anthropogenic sources
of organic carbon, such as sewage (e.g. Pearce 1998).
Concerns have been raised that current pollution
control policy aimed at reducing the organic carbon
content of sewage discharges might adversely affect
these bird populations.
Organic pollution of the sediments tends to result
in a reduction in biodiversity of benthic invertebrate
communities and the dominance by relatively few
taxa, usually annelid worms. While these taxa are
of some importance to fish and birds as food items,
the reduction in the variety of prey will result
in a decrease in the variety of fish and bird predators.
The contribution of POM to turbidity in the water
column can contribute to reduced production of phytoplankton,
macroalgae and other plants.
Potential effects on interest
features of European marine sites
Potential effects include:
- stimulation of heterotrophic bacterial production
in the water column and sediments;
- mineralisation of nutrients (N and P) from organic
matter and contribution towards the effects of
- organic enrichment of the water column and sediment
by stimulation of detritivore production, resulting
in a change in community composition in favour
of these animals and, possibly, their predators,
including sea birds;
- organic pollution of the water column and sediment
which can reduce the biodiversity of animal and
plant communities with adverse consequences for
fish and bird populations;
- increased oxygen demand in the water column
that can have sub-lethal and lethal effects on
fish populations, resulting in reductions in biodiversity
with possible consequences for fish eating birds;
- contribution to turbidity in the water column
with potential to suppress production of phytoplankton,
macroalgae and other submerged aquatic plants.