Chapter 6 Mass Balance of Nutrient Fluxes in Coastal Lagoons

Net mass balance analysis of material inputs to an ecosystem minus outputs from that system provides a measure of how that system is biogeochemically coupled with adjacent systems. Some materials, such as total salts (most often expressed as salinity), usually show little or no evidence of net uptake or release in coastal marine systems; such materials are said to be ‘conserved’ in the system. Materials which do show release are termed ‘nonconservative’. The dissolved nutrient elements phosphorus, nitrogen, and carbon are among the most conspicuous and ecologically important nonconservative materials in aquatic environments. This paper presents a methodology for evaluating nonconservative fluxes in coastal lagoons. The methodology, borrowed from studies of material fluxes in bays and estuaries, has apparently not been used in coastal lagoons. At steady state in a hydrographically simple system, any material, Y, which is conserved with respect to salinity will be distributed along a straight line on a graph of salinity versus Y. The position of the straight line is fixed by the composition of water at either salinity extreme of the system. Both temporal variation in end-member water composition and hydrographic complexity can cause deviations from this simple straight-line behavior. With proper attention to these factors, however, mixing models can be used to evaluate the degree of departure of any material from a simple conservative relationship with salinity. If a water budget is available for the system of interest, then the rate of net nonconservative flux can be calculated. The model has been most frequently applied to systems showing net freshwater input; the model is readily applicable and somewhat more robust in systems showing net evaporation. The mathematical derivation for this model is presented. Nonconservative fluxes of dissolved inorganic phosphorus, nitrogen, and phosphorus are largely related to primary production minus respiration (i.e., to net metabolism) in aquatic systems, although each of these elements has complications in its flux pathways. Nonconservative carbon flux is complicated by both calcium carbonate precipitation-dissolution reactions and gas exchange across the air-water interface. Nitrogen flux is complicated by nitrogen fixation and denitrification. Acknowledging that phosphorus flux can be associated with various abiotic sorption and precipitation reactions, we nevertheless consider nonconservative phosphorus flux to be the best measure of net ecosystem metabolism. Systems which import more dissolved phosphorus than they release are apparently net autotrophic; systems which export more dissolved phosphorus than they import are apparently net heterotrophic. There is very little annual-average information on net nutrient fluxes in coastal marine systems, but a number of studies describe the primary production and respiration of major system components. We can infer that most of these systems are likely to export dissolved phosphorus. We believe, based on analogy with estuaries and limited lagoonal data, that most coastal lagoons are likely to be net heterotrophic. This expectation requires further evaluation.