Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 86

Organic compounds are mineralized through both aerobic and anaerobic res-
piration processes. Aerobic respiration, which takes place in the surface sediment
layers (typically 0-5 mm depth), results in a rapid depletion of oxygen. In the
sediment, bacteria oxidize a significant fraction of the organic matter using terminal
electron acceptors other than oxygen (e.g., nitrate, manganese and iron compounds,
sulfate, and carbon dioxide). 8,29 The two dominant anaerobic processes are dissim-
ilatory sulfate reduction and methanogenesis (methane production). Generally,
sulfate reduction precedes methanogenesis because sulfate-reducing bacteria out-
compete methanogens for substrates. Freshwater has lower sulfate concentrations
(10-200

M) than estuarine water (30 mM). 22 Decomposition through sulfate
reduction occurs deeper in the sediment column (

µ

10 cm) and provides an addi-
tional source of NH 4 + . 26 Sulfate reduction, and subsequent inhibition of nitrification
and denitrification by HS − , should lead to enhanced ammonium regeneration during
summer, when sulfate reduction rates are high compared with those in winter. 22 In
all cases, the mineralization of organic nitrogen compounds results in the produc-
tion of NH 3 /NH 4 + .
All living matter contains nitrogenous macromolecules, which become available
to decomposer organisms upon the death of cells. Depending upon the structural
complexity of the organic matter, mineralization can either be a simple deamination
reaction or a complex series of metabolic steps involving a number of hydrolytic
enzymes. Thus, mineralization rates depend on the degradability of the organic
matter; i.e., whether it is labile or highly refractory. For example, seagrass detritus
that has 25-30% lignin containing fibers, has a lower mineralization rate than
phytoplankton cells, which contain more labile nitrogenous material. 8 Another
parameter affecting the mineralization rate of organic matter is temperature. 7 Sea-
sonal patterns of benthic nutrient regeneration generally exhibit strong summer
maxima, which correlate well with water temperature. The effects of temperature
can be represented by Arrhenius type expressions. 27
Mineralization of organic nitrogen plays a central role in nitrogen recycling in
coastal marine environments. Regeneration from the sediment regulates all produc-
tivity since inorganic nutrients are the limiting factors for primary production, 30 and
much of the primary production of many coastal marine systems is supported by
nutrient recycling rather than by nutrient inputs alone. 26 In shallow water ecosystems,
benthic recycling may account for 20-80% of the nitrogen requirements of the
phytoplankton. 8,27 Nixon 26 reported that nutrient inputs to Narragansett Bay, U.S.A.
(without being recycled) could support, at the most, only 24-50% of the annual
production, depending on the nutrient considered.
Ammonium produced during the deamination of organic nitrogen in sedi-
ment is not totally available to the primary producers; some of the ammonium
remains dissolved in interstitial water, some is adsorbed and buried into deeper
sediment, 7 some is consumed by benthic algae for cell synthesis, 13 and a fraction
undergoes nitrification in the surficial oxic zone of the sediment. 8 Denitrification
following nitrification produces gaseous forms of nitrogen (N 2 , N 2 O) essentially
unavailable to most coastal phytoplankton. 2,31 Thus, the coupled processes of
nitrification-denitrification represent a sink that shunts nitrogen away from recy-
cling pathways. 20

>