Environmental Engineering Reference
In-Depth Information
In lakes, as algae and other organisms die and settle to the bottom some N leaves the
epilimnion and is no longer available for N cycling in the top layers. The dark bottom
water layers of the lake (hypolimnion) and sediments are net heterotrophic and processes
such as decomposition, mineralization, immobilization, nitrification, and denitrification
dominate. As a result, the dark, cold, and often anoxic conditions promote different path-
ways of N cycling. The decomposition of organic matter and lack of photosynthesis drives
down oxygen concentrations in the water column and some lakes may have anoxic hypo-
limnia. The concentrations of dissolved nutrients in the bottom layers can be much higher
than in the surface layers because of high mineralization rates and low uptake rates. In
anoxic hypolimnia, concentrations of ammonium can be high because the low oxygen con-
ditions block nitrification. In aerobic hypolimnia, N transformations in the sediments are
controlled by the flow of oxygen and detritus from the water column, which leads to a
marked layering of redox conditions within the sediments. The surface layer of the sedi-
ment may be aerobic, with high rates of decomposition, mineralization, and nitrification.
High rates of oxygen consumption combined with low rates of diffusion can lead to the
development of anaerobic layers below the surface aerobic layer. If nitrification rates are
high, a supply of NO
3
2
can support a vigorous denitrification layer just below the surface
aerobic layer. This coupled nitrification
denitrification layering can make aquatic sedi-
ments significant “sinks” for N in the landscape, since much of the N that enters these sys-
tems is returned to the atmosphere. Beneath the denitrification layer, other anaerobic
processes (e.g., sulfate reduction, methanogenesis) dominate (see the Appendix). In strati-
fying lakes, the layers of water can mix completely during seasons when thermal stratifica-
tion of the layers breaks down (typically spring and fall). During these times, the bottom
nutrient-rich waters mix with the nutrient-poor surface waters and act to “reset” the sys-
tem, which can promote seasonal increases in primary production.
In oceans, similar depth-driven changes in light, settling, and decomposition can lead to
variable N transformations in different zones. In oceans, rates of primary production are
often N limited and the sources of N are fixation, runoff of N from rivers, zones of upwell-
ing, and internal cycling. The sunlit surface waters of the ocean (euphotic zone) contain pri-
mary producers (e.g., phytoplankton) and many heterotrophic organisms such as
zooplankton, fishes, marine mammals, and birds. Because particulate carbon (e.g., phyto-
plankton, marine snow, and fecal pellets) produced in the euphotic zone sinks, the supply
of nitrogen is greater than can be supplied by internal cycling alone (
Eppley and Peterson
1979
). The nitrogen available for primary producers in the euphotic zone comes from inter-
nal recycling and external inputs such as upwelling, N-fixation, and nitrogen from terrestrial
landscapes via river flow. In addition, ocean shelf sediments and oxygen minimum zones
associated with the decomposition of settled organic matter can promote higher levels of
anoxic N transformations (e.g., denitrification). The sinking of particulate matter from the
euphotic zone fuels heterotrophic activity and is a source of nitrogen to the deep ocean.
Movements of organisms that feed at depth and come to the surface (e.g., whales) can also
bring nutrients from deep zones to the photic zone.
Cycling of N in standing-water ecosystems can vary, and be disrupted by human activi-
ties in several ways. Stimulation of primary production by addition of nutrients (typically
phosphorus in freshwater, N in saltwater, but also often colimited by both N and phos-
phorus) can lead to algal blooms and altered ecosystem properties. For example, increases
