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.
Natural and anthropogenic processes accumulate surface-reactive solutes (i.e., certain organic and inorganic nutrients and toxicants) in bed sediment over multiple (even decadal) time scales. While oxic, the sediments retain the solutes, but as reducing conditions occur (on a daily or seasonal basis), the solutes may desorb. As these solutes repartition and remobilize (sometimes becoming dissolved and thus part of the porewater), the resulting concentration gradient in interstitial waters drives benthic flux. Restoration efforts to expedite flushing and hence improve pond water quality may eventually lead to reduced porewater gradients for contaminants and/or macronutrients. However, these desired changes might not be seen for many years, lagging far behind any potential point-source (e.g., sewage treatment effluent) improvements. The decades-long accumulation of solutes in pond sediments will continue to support vertical concentration gradients across the sediment-water interface and the resultant benthic flux, until either the solute concentrations above and below the sediment-water interface re-equilibrate, or deposition of sufficient new sediment (with decreased solute concentrations of concern) diminishes the concentration gradient. In other words, nutrients sources in the sediment cannot be ignored when resource managers are proposing long-term water-quality improvements. Sediments may supply nutrients to the system, causing otherwise unexplained slow progress or even regression of water quality to undesirable conditions.
Prior to the mid-19th century, the San Francisco Bay estuary was surrounded by approximately 80,000 hectares of tidal wetlands. Beginning with the California Gold Rush in 1849, nearly 90% of these wetlands [
Specifically, after decades of use as evaporation ponds, the system would be gradually transitioned to allow steady flows through the ponds. The monitoring of water quality as part of the restoration process was given tremendous attention and resources. These now evolving ecosystems support terrestrial, aquatic and avian wildlife communities, as well as recreational areas of public interest and concern. In addition, pelagic and benthic algal communities, while less charismatic, form the base of the system’s food web. The nutrients and toxicants consumed by these organisms can be magnified up the food chain [
The primary objective of this study (from 2010 to 2012) is to determine if the sediment in decades-old salt ponds undergoing restoration to wetlands can represent a significant source or sink of nutrients for the overlying water relative to other solute-transport processes. The approach used here includes evaluating the diffusive benthic flux using a mechanical porewater sampler; a benthic flux comparison to an open water estuary; comparison between advective and benthic flux; and the role that macroinvertebrates may have on bioturbation to augment benthic solute flux.
The Alviso Salt pond system, is located about 10 miles northwest of San Jose, California, and sits at the south end of the southernmost portion of San Francisco Bay, CA (
Sampling site “Inlet” (
Secondary sites Discharge, Shallow 2 and Algal (located where 1 - 10 cm thick algal mats had been observed) were included for select analyses to account for biological heterogeneity in the pond (
A non-metallic pore-water profiler [
Concurrent with profiler deployments at Deep and Inlet sites, water-column samples were collected and continuous measurements of water-column pH, temperature, dissolved oxygen, and salinity were recorded at 15-minute intervals [
In addition to porewater sampling, weekly grab samples were collected from just below the water surface during the middle to end of the Mediterranean dry seasons of 2010, 2011 and 2012 at all sites. These samples were filtered and analyzed for macronutrients (dissolved ammonia, SRP, nitrate, nitrite) to determine whether concentrations were depleted during bloom events, and also for trace metals. In 2011 and 2012, weekly samples were collected in dark brown, opaque bottles for water-column chlorophyll analysis. Samples were kept in darkness for the brief trip to a laboratory where they were glass-fiber filtered under a vacuum pressure of less than 5 psi to collect algal cells. Filters were extracted in 90% acetone for 24 hours at <32˚F in darkness and analyzed for chlorophyll by fluorometry [
To estimate advection of nutrients in and out of the system, current velocities were measured and water-col- umn grab samples were collected at the “Inlet” and “Discharge” sites every 2 hours over a 24-hour cycle for both a neap and a spring tidal cycle [
Sediment cores (10 cm deep, 10 cm diameter) were collected in both sampling years at Inlet, Algal, Discharge, and Shallow 2 (
Sediment oxygen demand was determined by two methods: porewater diffusion and core incubation. Dissolved oxygen demand was difficult to accurately measure using porewater diffusion calculations due to the combination of oxygen saturated overlying water and sediment that was anoxic at 1 cm depth. Thus, acrylic tubes were used to collect approximately 10 cm deep sediment cores from which overlying water was sampled at hourly intervals. Samples were analyzed for dissolved oxygen, and oxygen demand was calculated based on the concentration time series [
Dissolved (0.2-µm filtered) orthophosphate, ammonia, nitrate, nitrite and silica concentrations in both water- column and pore-water samples were determined in triplicate by low-volume, discrete single cuvette, spectrophotometric methods [
Each sediment core was sieved at 500 µm for macroinvertebrates and sorted completely. Magnifications from 7× to 45× were used to sort and identify organisms, and macroinvertebrates were identified to Order or Class.
Diffusive solute flux across the sediment-water interface was calculated using solute concentrations of the overlying water and the first few centimeters of porewater using Fick’s Law [
The diffusive flux (Ji in units of µmoles of solute i per m2-hr) was calculated by the equation:
where
Di,T is the diffusion coefficient of solute i at temperature T in cm2/s,
φ is the dimensionless sediment porosity, and
dCi/dz is the concentration gradient for solute i in the vertical direction in µmoles/L/cm.
These benthic-flux calculations (via solute-specific diffusion coefficients) assume that diffusion controls benthic flux [
Values were normalized when necessary using log10 transformation prior to statistical comparisons. Comparisons between sites or dates, or flux results from other studies, were assessed using paired t-tests. Differences are considered significant when p < 0.05, where p represents the probability that the differences are not significant. Data reported are means ± SD.
Benthic flux of SRP (orthophosphate) was frequently positive (i.e., out of the sediment into the water column,
Averages for SRP benthic fluxes in the pond were greater than those observed in the southern portion of the adjacent open-water estuary (San Francisco Bay) using a core-incubation technique which integrates the effects of bioturbation and bioirrigation [
within the pond exhibited a mean of 4.9 ± 1.4 g-P/m2/yr (n = 54), suggesting that the SRP flux in the pond system is much greater than that in the nearby open estuary. The seasonal and annual temporal variability in SRP flux in the restoration pond is also greater than reported for the adjacent estuary. Weekly water-column sampling never showed orthophosphate to be depleted.
Dissolved ammonia benthic flux was consistently positive for all sampling dates, with significant within-pond variability. Compared to the mean of 3.1 ± 1.9 g-N/m2/yr (n = 18) reported for the adjacent estuary [
With the absence of any measurable concentration gradient, dissolved-nitrate fluxes were consistently negligible in the pond (<0.1 g-N/m2/yr), in contrast to variable but generally positive nitrate fluxes reported for the adjacent estuary (mean of 2.7 ± 2.2 g-N/m2/yr; n = 18) [
Dissolved-silica fluxes in the restoration pond exhibited a mean of 92 ± 38 g-SiO2/m2/yr (n = 53). The adjacent estuary [
The restoration pond exhibited orthophosphate and ammonia that are significantly larger than those previously reported for southern San Francisco Bay. The combination of these fluxes, shallow water, moderate temperatures and abundant sunlight are consistent with observed eutrophic conditions [
To compare the relative importance of allochthonous and autochthonous sources of nutrients to the pond, both advective and benthic fluxes were scaled into kg/year units (based on the 2.27 km2 pond surface area). The mean benthic flux of ammonia over the period of study was about 49,900 ± 13,000 kg-N/yr. Advective flux estimates from the external exchange over the tidal cycle ranged from −50 to 1500 kg-N/yr of ammonia (
In contrast to ammonia, the average benthic flux of SRP, 11,100 ± 3200 kg-P/yr, was below, but in the same order of magnitude as the advective flux range of 21,500 to 30,000 kg-P/yr. Although allochthonous SRP sources to the pond appear to dominate, the benthic SRP source should not be disregarded, especially if non- diffusive processes contribute significantly to the availability of SRP to the water-column.
The average mass ratio of nitrogen to phosphorous in the water column over all sampling sites and dates is 1.0 ± 0.5 or 7-times lower than the Redfield N:P ratio in phytoplankton of 7.2:1.0, indicating the possibility of N limitation. However, the reported presence of Anabaenopsis sp. and Anabaena sp., both N-fixing cyanobacteria [
Within the A3W pond system, autotrophic activity can generate benthic algal mats that are centimeters thick [
Bioturbation and bioirrigation by macroinvertebrates can potentially augment solute flux across the sediment-
water interface. Unlike other benthic flux studies where benthic organisms were absent [
Nutrients in shallow, estuarine, managed ponds, similar to shallow lakes, are influenced more by their benthic sources than deep water bodies due, at least in part, to the lower ratio of volume to bed-surface area. Benthic flux, even when calculated only from diffusion, can represent an important and potentially dominant nutrient source to the water column. This nutrient source should be regularly considered along with the focus on water flow through the system of ponds (i.e., advective transport). Despite management efforts to improve hydrologic exchange between the pond and adjacent estuary, it is likely that any return to natural conditions will take many years, possibly decades, similar to the time scales of solute accumulation in the pond beds.
As this large pond restoration project ends its first decade, we have only begun to understand the biogeochemical implications of restoration actions. However, these results do warrant consideration for restoration of any lentic system experiencing a legacy of contaminant deposition and accumulation, regardless of the mitigation strategy adopted.
The authors are grateful for critical logistical support from J. Chiu, and personnel from the U.S. Geological Survey (USGS) including Stacy Moskal, Lacy Smith, Greg Shellenbarger and Francis Parchaso. Thanks also go to Melisa Helton of the Don Edwards San Francisco Bay National Wildlife Refuge, for their project support. The U.S. Fish and Wildlife Service, USGS WERC in Vallejo, California, USGS San Francisco Bay Priority Ecosystems Study, USGS Toxic Substances Hydrology Program, San Francisco Regional Water Quality Control Board and the National Research Program within the USGS Water Mission Area are also acknowledged for support of this work.
Brent R. Topping,James S. Kuwabara,James L. Carter,Krista K. Garrett,Eric Mruz,Sarah Piotter,John Y. Takekawa, (2016) Effects of Salt Pond Restoration on Benthic Flux: Sediment as a Source of Nutrients to the Water Column. Journal of Environmental Protection,07,1064-1071. doi: 10.4236/jep.2016.77095