Week 3 - Eutrophication Flashcards
Patterns in marine primary productivity
High coastal productivity is
largely driven by the supply of nutrients from terrestrial sources and coastal up-welling of
nutrients from deep ocean environments. It is particularly high in large harbours and
enclosed seas where nutrient input and retention is highest. The productivity of the open
ocean is extremely low. Globally, marine productivity is therefore very sensitive to
changes in nutrient supply from land. The highest human populations and greatest levels
of land modification have occurred in estuarine and coastal environments: sewage
discharge, land clearing, coastal industries, urban run-off and agriculture have had
profound impacts on coastal marine environments. Global productivity will also be
sensitive to ocean warming and associated changes in ocean circulation that may reduce
up-welling along temperate coastlines.
Eutrophication
Eutrophication is the biological process initiated by excessive nutrient enrichment (Fig. 6).
In general, eutrophication is initially associated with an increase in primary production. If
slight, this might not represent a problem. However, for some habitats such as coral reefs,
organisms are adapted to low nutrient levels and cannot deal with an elevation in nutrient
levels. As eutrophication proceeds, increasing productivity is followed by a change in plant
species composition, as phytoplankton increases, but cuts out the light available for
macrophytic plants attached to the substrate. Primary productivity shifts from the benthos
to the water column. Eventually, very dense, often toxic blooms of planktonic organisms
occur. Bacterial metabolism increases as benthic plants die, leading to conditions of
hypoxia (low oxygen) and in extreme cases, anoxia (oxygen too low to support animals =
dead zones).
Dead zones
Dead zones—areas of bottom waters too oxygen depleted to support most ocean life—are
spreading, dotting nearly the entire east and south coasts of USA, north coast of Europe
and Baltic (Fig. 11). According to a study in Science (Diaz & Rosenberg 2008), there were
~400 identified dead zones worldwide in 2008, up from 49 in the 1960s—and the world’s
largest dead zone remains the Baltic Sea, whose bottom waters now lack oxygen yearround. You can see the greatest concentration of dead zones in the northern hemisphere,
associated with highly industrialised countries and enclosed seas.
Eutrophication and the global impact on seagrass ecosystems
Seagrasses are aquatic angiosperms all having a well-developed anchoring system and
the ability to complete the life cycle fully submerged in sea water.
World-wide there are 12 or so genera (~ 57 species) (Fig. 17). However, they often form
monospecific stands over huge areas and support a huge biodiversity of marine
invertebrates, fishes and other vertebrates.
It is easy to demonstrate bottom-up effects of seagrass on these other components of
seagrass ecosystems. For example, the mean annual yield of panaeid shrimp caught
inshore is closely correlated with the area of vegetated estuarine habitat, so any loss of
seagrass area will result in a predictable loss of fisheries production (Fig. 22).
How does eutrophication and suspended sediments interact to bring about seagrass
decline? (Fig. 27). Nutrients and suspended sediments reduce light penetration which
reduces the growth of seagrass. The nutrients, however, promote the growth of
phytoplankton, epiphytes and/or macro-algae at the expense of seagrass. This further
reduces light penetration, to the point where the seagrass can die. Once the seagrass
dies, the epiphytes and macro-algae may also be eliminated. So there are three factors
which reduce the amount of light striking the seagrass blades (Fig. 27): (1) Suspended
solids (2) dissolved nutrients. (3) Phytoplankton, and (4) Epiphytes and macroalgae.
Fig. 28 shows the typical decline in seagrass biomass and depth range as a function of
increasing nutrient input from the watershed. This figure shows the typical effect of
nutrients on epiphyte loads. Increasing nutrients can result in epiphyte loads reaching
200-400% of the actual seagrass weight.
What are five important structural and trophic functions of seagrass beds
(1) High primary productivity: measurements range from 500 – 4,000 C/m2/yr for
tropical Thalassia testudinum. This is equivalent to many cultivated crops such as corn or
rice ~500C/m2/yr.
(2) Nutrient cycling: nutrient cycles in seagrass beds are complex as seagrass can
utilize nutrients from the sediments and/or the water column. N, P, C, and S are converted
by seagrass into forms that are usable by other organisms.
(3) Food resource: A number of threatened species feed directly on seagrass (e.g.
green turtles, dugong, manatee (Fig. 21). Few fish and macro-invertebrates feed directly
on seagrass. However, they probably form a more important part of the detrital food chain.
Microorganisms consume these macrophytes and detritus-feeding animals derive their
nutrition from feeding on micro-organisms associated with dead plant matter.
(4) Stabilize sediments – roots and rhizomes form matrices with sediments and the
binding prevents erosion. Once seagrass has been thinned or damaged and the
sediments have been mobilized, it can be difficult for seagrass to re-establish.
(5) Shelter, nursery and breeding grounds for many invertebrates and fishes, including
many commercially important forms e.g. tiger prawns. It is almost an article of faith that
seagrass beds are important nursery sites for coastal fishes and maintain fishery
production.
Eutrophication and the global impact on coral reef ecosystems
Sedimentation as a result of land clearing and dredging has a number of well-known
effects on corals. Increased turbidity from sediment suspension can reduce light which
can reduce coral growth or cause mortality. Small amounts of sediment can reduce
growth as corals put energy into removing the sediment. Large amounts of sediment
smother corals, increasing mortality rates and inhibiting future settlement. Increasing
amounts of sediment can reduce coral cover and diversity through a variety of these
mechanisms (Fig. 34).
The most well known example of nutrient enrichment comes from Kaneohe Bay, Oahu,
Hawaii. This map shows the extent of coral death and replacement by the alga
Dictyosphaeria as a long-term response to dredging activities and sewage discharge into
an enclosed bay (Fig. 35).
Potential solutions
The first point is that reducing nutrients and sediment loads can lead to ecosystem
recovery, provided other factors are in place, such as protection of herbivores and large
predators. Remedial actions that target both ends of the food chain have the best chance
of success. In industrialised countries, catchment regulations, such as strict discharge
controls are important. Tertiary sewage treatment is a necessity. A reduction in the use of
fertilizers, combined with methods of reducing direct loss of N and P based fertilizers will
make an immediate different. Vegetation buffers around rivers and creeks should be
enforced. We need to halt remaining clearing of natural forests and reduce dependence
on monocultures, e.g. sugar, oil palm.
