TITLE:
Effects of Salt Pond Restoration on Benthic Flux: Sediment as a Source of Nutrients to the Water Column
AUTHORS:
Brent R. Topping, James S. Kuwabara, James L. Carter, Krista K. Garrett, Eric Mruz, Sarah Piotter, John Y. Takekawa
KEYWORDS:
Benthic Flux, Nutrients, Algal Blooms, Restoration
JOURNAL NAME:
Journal of Environmental Protection,
Vol.7 No.7,
June
22,
2016
ABSTRACT:
Understanding nutrient flux between the benthos and the overlying water
(benthic flux) is critical to restoration of water quality and biological
resources because it can represent a major source of nutrients to the water
column. Extensive water management commenced in the San Francisco Bay,
Beginning around 1850, San Francisco Bay wetlands were converted to salt ponds
and mined extensively for more than a century. Long-term (decadal) salt pond
restoration efforts began in 2003. A patented device for sampling porewater at
varying depths, to calculate the gradient, was employed between 2010 and 2012.
Within the former ponds, the benthic flux of soluble reactive phosphorus and
that of dissolved ammonia were consistently positive (i.e., moving out of the
sediment into the water column). The lack of measurable nitrate or nitrite
concentration gradients across the sediment-water interface suggested
negligible fluxes for dissolved nitrate and nitrite. The dominance of ammonia
in the porewater indicated anoxic sediment conditions, even at only 1 cm depth,
which is consistent with the observed, elevated sediment oxygen demand. Nearby
open-estuary sediments showed much lower benthic flux values for nutrients
than the salt ponds under resortation. Allochthonous solute transport provides
a nutrient advective flux for comparison to benthic flux. For ammonia, averaged
for all sites and dates, benthic flux was about 80,000 kg/year, well above the
advective flux range of -50 to 1500 kg/year, with much of the variability
depending on the tidal cycle. By contrast, the average benthic flux of soluble
reactive phosphorus was about 12,000 kg/year, of significant magnitude, but
less than the advective flux range of 21,500 to 30,000 kg/year. These benthic
flux estimates, based on solute diffusion across the sediment-water interface,
reveal a significant nutrient source to the water column of the pond which
stimulates algal blooms (often autotrophic). This benthic source may be
augmented further by bioturbation, bioirrigation and episodic sediment
resuspension events.