Nutrient Cycling

nutrients: sources and transformations

The largest mass of nutrients is delivered by incoming tidal waters and, for example, a square meter of marsh inundated twice a day by 25 cm of tidal water carrying 0.5 mg dissolved inorganic N • L-1 has a potential supply of 250 mg N • m-2 • d-1. At least a portion of this potential N supply is retained within some marshes since ebb-tide concentrations of nitrate were roughly 1 mg NO3 • L-1, lower than mean nitrate concentrations in the mainstem, where NO3 is 2.5 mg NO3 • L-1 (Lampman, Caraco, and Cole, 1999). Phosphorus (P) concentrations in ebb-tide waters range from 40 to 80 |g PO4 • L-1 and are not demonstrably lower than in the main-stem (PO4 ~ 60 |g• L-1; Lampman et al., 1999). Given the significant plant demand for PO4, it suggests there must be reasonably large sources of PO4 to balance plant removal and maintain rough equality in flood versus ebb tide concentrations. This P source may be from tributary inputs since many tributaries (for example, Stony Creek and Saw Kill in the Tivoli Bays) receive treated sewage leading to greatly elevated P concentrations. Sediment P release is another likely candidate for this source of P since porewater concentrations are quite high (see below). If porewaters can bypass the surface oxidized layer where P is effectively scavenged (Vernon, 2002), the large reservoir of inorganic P in deeper sediment layers (ultimately derived from decay of organic materials) may contribute to the net efflux of P from Hudson marshes. There is essentially no information on sources or exchanges of organic nutrients (dissolved organic N, dissolved organic P, etc.) although some Hudson marshes act as minor sources of dissolved organic C to the mainstem (Findlay et al., 1998).

Tributary inputs of nutrients to Hudson River marshes vary greatly among sites due to both variation in size of tributary relative to area of marsh and variation in nutrient concentration, the latter of which seems related to agricultural lands and human population (Parsons and Lovett, 1992; Nieder, unpublished). Tributaries are only occasionally a large source compared to tidal waters; for instance, at Tivoli North Bay, Stony Creek has a relatively high dissolved inorganic N concentration (1998 mean >2 mg NO3 • L-1; Hudson River National Estuarine Research Reserve, unpublished data) and a modest discharge (~100 • L s-1 summer) resulting in a potential contribution of about 40 mg N • m-2 d-1 (assuming an intertidal marsh area of 1,000,000 m2 based on area of cattail plus purple loosestrife), much less than the potential tidal water contribution (250 mgN • m2 • d).

Porewater nutrients represent a large potential pool and more importantly are the proximate source of nutrients for the emergent vascular plants. Typical growing season porewater concentrations are 0.25-1 mg ammonium • L-1 with frequently low nitrate concentrations of 0.1 mgN • L-1 or less. Porewater phosphate is typically high with concentrations >1-2 mg PO4-P • L-1 and significant variability among plant species (Templer et al., 1998). Porewater nutrient concentrations vary among marsh types, with two to three times higher concentrations in enclosed marshes (NH4-N = 0.5 mg • L-1; PO4 = 9 mg • L-1) versus sheltered or fringe (0.22 and 1.64 mg • L-1, respectively). Phosphate was significantly, positively (r = 0.68) related to sediment organic content, which varies among marsh types. Plant demand for porewater nutrients is large relative to porewater nutrient standing stocks and plant uptake is sufficient to turn over porewater pools several times during the growing season.

nutrient budgets

For Tivoli North Bay there are sufficient data to construct a rough annual N budget. The site is a net sink for N as indicated by the lower ebb-tide (0.23 mg NO3-N• L-1) than flood-tide (0.56 mg NO3-N • L-1) concentrations. Net tidal input is estimated as the mean concentration difference X tidal volume/marsh area = 0.33 mg N • L-1 X 250 L • m-2 = net loss of 3.3 mg N • m-2 per tidal cycle during the summer. Assuming 100 days of N removal per year (non-summer ebb nitrate concentrations are not significantly below mainstem values) this yields a net removal from tidal waters of 14 g N • m-2 • yr. Atmospheric deposition directly to the marsh surface is about 1 g N • m-2 • yr using a regional estimate of deposition (10 kg N ha yr). The tributary input from Stony Creek is about 24 g N • m-2 • yr (1994 means: annual mean flow 1,450 L • s-1 X mean concentration 3.34 mg NO3 • L-1, total area = 1.4 x 106 m2; Nieder, unpublished). The removal of NO3 from tidal waters is probably due to a combination of plant and microbial uptake and denitrification. Plant demand for N is fairly large with, for example, 5 g • N m-2 in aboveground biomass of narrowleaf cattail (Templer, Findlay, and Wigand, 1998) and over 10 g N • m-2 in common reed (Findlay et al.,

Table 20.5. Topics for future study of Hudson River tidal wetlands

Component

Question

Implications

Water control structures

Would enlargement reverse

Vegetation and habitat

(trestles, culverts)

sedimentation?

management; restoration

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