Plant Bound Nutrient Transport. Mass Transport In Estuaries And Lagoons

In many regions of the world external nutrient loading to estuaries has recently been decreasing. These estuaries are only beginning to adjust to this change. The change is visible as better water quality. The period where the phytoplankton is growth limited has become longer, resulting in a better light climate at the bottom with the potential for recolonization of benthic macrophytes. This phase of recovery often shows a nonlinear behaviour and can be longer than expected (Borum 1996, 1997). National monitoring programs have been developed with the purpose of following the expected recovery phase. Nutrient loading, water quality and the export of nutrients from these systems are the main focus of this monitoring. For this reason, increasing attention has been paid to nutrient mass balances at the outer boundary of lagoons and estuaries. The measurements of estuarine mass balances have traditionally been limited to dissolved inorganic nutrients (ammonia, nitrite, nitrate, phosphate) and fine particulate matter fraction trapped on filters (e.g. glass fiber filters). Often no distinction is made between living and detrital particulate matter. This chapter will show that most of these nutrient mass balances are incomplete. The reason is that in shallow productive micro- and meso-tidal estuaries plant bound nutrient transport is essential for the nutrient mass balance (Flindt et al. 1997a, 1999, Salomonsen et al. 1997, 1999). Why is plant bound nutrient transport not included in the mass balances? A part of the explanation is that the plant matter accumulates at the bottom – and the transport afterward takes place as bedload transport. Although Odum et al. (1979) pointed out that import/export studies have failed to account for transport of particulate loads on or near the bed of estuaries, this transport is still often neglected. Another explanation is that when a piece of a macroalgae is infrequently trapped in a water sample of a few litres, this piece will usually be removed, because the sample is considered non-representative and would introduce a high variability among the samples. The inclusion of a 50 g wwt piece of Ulva in a 5 l water sample may increase the total nitrogen and total phosphorus concentration by a factor of 50.