[Show abstract][Hide abstract] ABSTRACT: Primary production was measured from 1992–2010 in Massachusetts Bay and just outside Boston Harbor for the Massachusetts Water Resources Authority’s outfall monitoring program. In 2003, annual primary production decreased by 221–278 g C m^-2 year^-1, with decreased rates continuing through 2010. Based on a conceptual model, oceanographic and meteorological variables were analysed with production rates to determine if concurrent environmental changes were responsible for the reduced primary production in Massachusetts Bay. Results indicated that a stronger influx of low salinity water from the Western Maine Coastal Current (WMCC) in recent years might be responsible for the decreases. The WMCC appeared to have become fresher due to increased river discharge in the western Gulf of Maine. Northeasterly winds in recent years promoted the WMCC intrusion into Massachusetts Bay. Correlation between primary production and surface salinities suggested an impact of the WMCC on production rates. We hypothesized that increased stratification resulted in reduced vertical mixing and reduced nutrient concentrations in surface waters for phytoplankton growth. However, no significant correlations were observed between the annual primary production and nutrient concentrations in Massachusetts Bay. Reduced production rates in Massachusetts Bay have, however, been associated with reduced zooplankton abundances, benthic ammonium fluxes and sediment oxygen demand in summer months.
[Show abstract][Hide abstract] ABSTRACT: We identified eight Panamanian watersheds in which conversion from wet tropical forest to pastures differed and assessed the effects of degree of deforestation, and down-estuary transformations, on the suspended particulate matter discharged from the watersheds, entering, traversing through mangrove estuaries, and emerging into coastal waters. Deforested watersheds discharged larger concentrations of suspended particulate matter, with lower % C and N, higher mineral content, and heavier isotopic signatures into fresh reaches of estuaries. Down-estuary, sediment entrainment increased non-organic content of particulates, and watershed-derived imprints of deforestation on composition of particulate matter were mostly erased by within-estuary transformations. Isotopic signatures of C, N, and S in particulate matter demonstrated strong land-sea couplings, and indicated that the direction of the coupling was asymmetrical, with terrestrial and estuarine sources delivering particulate materials to coastal waters and sediments. Mangrove estuaries therefore both act as powerful modulators of human activities on land, while also exporting particulate materials to sea.
[Show abstract][Hide abstract] ABSTRACT: Benthic respiration, sediment–water nutrient fluxes, denitrification and dissimilatory nitrate reduction to ammonium (DNRA)
were measured in the upper section of the Parker River Estuary from 1993 to 2006. This site experiences large changes in salinity
over both short and long time scales. Sediment respiration ranged from 6 to 52mmol m−2 day−1 and was largely controlled by temperature. Nutrient fluxes were dominated by ammonium fluxes, which ranged from a small uptake
of −0.3 to an efflux of over 8.2mmol N m−2 day−1. Ammonium fluxes were most highly correlated with salinity and laboratory experiments demonstrated that ammonium fluxes increased
when salinity increased. The seasonal pattern of DNRA closely followed salinity. DNRA rates were extremely low in March, less
than 0.1mmol m−2 day−1, but increased to 2.0mmol m−2 day−1 in August. In contrast, denitrification rates were inversely related to salinity, ranging from 1mmol m−2 day−1 during the spring and fall to less than 0.2mmol m−2 day−1 in late summer. Salinity appears to exert a major control on the nitrogen cycle at this site, and partially decouples sediment
ammonium fluxes from organic matter decomposition.
KeywordsBenthic respiration-Benthic fluxes-Salinity-Ammonium-Denitrification-Dissimilatory nitrate reduction to ammonium (DNRA)-Estuary-Sediments
Estuaries and Coasts 01/2010; 33(5):1054-1068. · 2.56 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We examined the effects of seasonal salinity changes on sediment ammonium (NH4
+) adsorption and exchange across the sediment–water interface in the Parker River Estuary, by means of seasonal field sampling,
laboratory adsorption experiments, and modeling. The fraction of dissolved NH4
+ relative to adsorbed NH4
+ in oligohaline sediments rose significantly with increased pore water salinity over the season. Laboratory experiments demonstrated
that small (∼3) increases in salinity from freshwater conditions had the greatest effect on NH4
+ adsorption by reducing the exchangeable pool from 69% to 14% of the total NH4
+ in the upper estuary sediments that experience large (0–20) seasonal salinity shifts. NH4
+ dynamics did not appear to be significantly affected by salinity in sediments of the lower estuary where salinities under
10 were not measured. We further assessed the importance of salinity-mediated desorption by constructing a simple mechanistic
numerical model for pore water chloride and NH4
+ diffusion for sediments of the upper estuary. The model predicted pore water salinity and NH4
+ profiles that fit measured profiles very well and described a seasonal pattern of NH4
+ flux from the sediment that was significantly affected by salinity. The model demonstrated that changes in salinity on several
timescales (tidally, seasonally, and annually) can significantly alter the magnitude and timing of NH4
+ release from the sediments. Salinity-mediated desorption and fluxes of NH4
+ from sediments in the upper estuary can be of similar magnitude to rates of organic nitrogen mineralization and may therefore
be important in supporting estuarine productivity when watershed inputs of N are low.
KeywordsSediments-Ammonium-Adsorption-Parker River estuary-Salinity-Estuary
Estuaries and Coasts 01/2010; 33(4):985-1003. · 2.56 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The relationship between ammonia-oxidizing bacteria (AOB) and potential nitrification rates was examined along a salinity gradient in a New England estuary in spring and late summer over 3 years. Ammonia-oxidizing bacteria abundance was estimated by measuring gene copies of the ammonia monooxygenase catalytic subunit (amoA) using real-time polymerase chain reaction. Ammonia-oxidizing bacteria abundance ranged from below detection to 6.0 x 10(7)amoA copies (gdw sediment)(-1). Mean potential nitrification rates ranged from 0.5 to 186.5 nmol N (gdw sediment)(-1) day(-1). Both AOB abundance and potential rates were significantly higher in spring than late summer. Correlations between rates and abundance varied significantly among sites, but showed site-specific ammonia oxidation kinetics related to AOB community structure. The effect of salinity on potential nitrification rates was evaluated by incubating sediment from each site under four salinity conditions (0, 5, 10 and 30 psu). At all sites, rates were generally highest in the intermediate salinity treatments, but rates at the upstream site were inhibited at high salinity, while rates at the two downstream sites were inhibited at the lowest salinity. Although salinity appears to be an important factor in determining AOB distribution, it may not be the primary factor as AOB exhibited a broad range of salinity tolerance in our experiments. Our results indicate that there are significant differences in abundance and community composition of AOB along the salinity gradient, and the differences are reflected in community function.
[Show abstract][Hide abstract] ABSTRACT: Benthic metabolism and nutrient cycling were examined in depositional sediments of Broad Sound, Massachusetts Bay, Stellwagen Basin and Cape Cod Bay between 1990 and 1994 in water 16 to 76 in deep. Bottom water nitrate and dissolved oxygen concentrations were typically <10 muM and >70% of saturation. Sediment organic content was uniformly low at all sites ranging from 1.3 to 2.1 % carbon and 0.1 to 0.3 % nitrogen. Sediment chlorophyll a and phaeopigment concentrations averaged ca 2 and 15 ug cm(-3) respectively. Porewater nutrients and sediment-to-water fluxes of O-2, dissolved inorganic carbon (DIC), NH4+, NO2- + NO3-, urea, PO43-, N2O and dissolved silicon (DSi), were measured in cores from 4 stations, 4 to 6 times a year, for 2 yr, in order to examine seasonal and annual patterns. An additional 8 sites were examined over several years in late summer in order to examine spatial patterns. Average benthic community respiration ranged from 10.6 to 14.3 mmol O-2 m(-2) d(-1) for 12.1 to 29.8 mmol C m(-2-)d(-1). Within regions, respiration was highest in Broad Sound, followed by Massachusetts Bay, Cape Cod Bay and Stellwagen Basin. Sediments were sources of inorganic N. Annual average dissolved inorganic nitrogen (DIN) fluxes ranged from 0.5 to 1.8 mmol N m(-2) d(-1). For all stations, the flux of DIN was dominated by NH4+. The importance of NO3- fluxes relative to NH4+ increased with increasing depth. For example, in late summer the relative importance of NO3- fluxes increased from 0 to 44 % of total DIN flux between Broad Sound (<24 m) and Stellwagen Basin (75 m). Fluxes of N2O and urea were always extremely small (<0.1 % of DIN). Seasonal cycles, which in general tracked bottom-water temperature and phytoplankton sedimentation, were very strong for respiration (O-2 and DIC) but not for fluxes of NO3-, urea, N2O, and PO43-. Annual variation in benthic respiration was ca 25 %, similar to that for primary production. Inorganic nutrient fluxes often varied in excess of 100 % between years. Sediment mixing or porewater advection is thought to contribute to high nutrient flux variability. C, N, and P flux stoichiometry deviated substantially from the Redfield ratios, suggesting strong phosphorus retention by sediments and large nitrogen removal via denitrification. DSi fluxes were relatively high, which we attribute to focusing of biogenic material in these depositional sediments. Denitrification exceeded total DIN flux, averaging 1.9 and 1.3 mmol N m(-2) d(-1) in Massachusetts Bay and Stellwagen Basin respectively. The percentage of remineralized N that was denitrified increased with increasing water depth (from 60 to 72 % for 35 and 75 m water). Denitrification was supported almost exclusively by coupled nitrification and denitrification. The benthic community decomposed 12 to 22 % of overlying water primary production but only provided 3 to 8 % of the phytoplankton inorganic N requirement.
Marine Ecology Progress Series 01/2001; 224:1-19. · 2.55 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Benthic metabolism and nutrient exchange across the sediment-water interface were examined over an annual cycle at four sites
along a freshwater to marine transect in the Parker River-Plum Island Sound estuary in northeastern Massachusetts, U.S. Sediment
organic carbon content was highest at the freshwater site (10.3%) and decreased along the salinity gradient to 0.2% in the
sandy sediments at the marine end of the estuary. C:N ratios were highest in the mid estuary (23:1) and lowest near the sea
(11:1). Chlorophyll a in the surface sediments was high along the entire length of the estuary (39–57 mg chlorophyll a m−2) but especially so in the sandy marine sediments (172 mg chlorophyll a m−2). Chlorophyll a to phaeophytin ratios suggested most chlorophyll is detrital, except at the sandy marine site. Porewater sulfide values varied
seasonally and between sites, reflecting both changes in sulfate availability as overlying water salinity changed and sediment
metabolism. Patterns of sediment redox potential followed those of sulfide. Porewater profiles of inorganic N and P reflected
strong seasonal patterns in remineralization, accumulation, and release. Highest porewater NH4
+ values were found in upper and mid estuarine sediments, occasionally exceeding 1 mM N. Porewater nitrate was frequently absent,
except in the sandy marine sediments where concentrations of 8 μM were often observed. Annual average respiration was lowest
at the marine site (13 mmol O2 m−2 d−1 and 21 mmol TCO2 m−2 d−1) and highest in the mid estuary (130 mmol O2 m−2 d−1 and 170 mmol TCO2 m−2 d−1) where clam densities were also high. N2O and CH4 fluxes were low at all stations throughout the year: Over the course, of a year, sediments varied from being sources to sinks
of dissolved organic C and N, with the overall spatial pattern related closely to sediment organic content. There was little
correlation between PO4
3− flux and metabolism, which we attribute to geochemical processes. At the two sites having the lowest salinities, PO4
3− flux was directed into the sediments. On average, between 22% and 32% of total system metabolism was attributable to the
benthos. The mid estuary site was an exception, as benthic metabolism accounted for 95% of the total, which is attributable
to high densities of filter-feeding clams. Benthic remineralization supplied from less than 1% to over 190% of the N requirements
and 0% to 21% of the P requirements of primary producers in this system. Estimates of denitrification calculated from stoichiometry
of C and N fluxes ranged from 0% for the upper and mid estuary site to 35% for the freshwater site to 100% of sediment organic
N remineralization at the marine site. We hypothesize that low values in the upper and mid estuary are attributable to enhanced
+ fluxes during summer due to desorption of exchangeable ammonium from rising porewater salinity. NH4
+ desorption during summer may be a mechanism that maintains high rates of pelagic primary production at a time of low inorganic
N inputs from the watershed.
Estuaries and Coasts 01/1999; 22(4):863-881. · 2.56 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Using stable isotopes, we assessed the effects of long-term sewage inputs within Boston Harbor and extending into adjacent Massachusetts Bay. We used nitrogen and sulfur stable isotopes (δ15N and δ34S) to distinguish between sources of these elements to sediments, particulate organic matter, algae, and animals. The isotope data revealed the widespread presence of sewage-derived particulate and dissolved materials. Incorporation of sewage-derived effluent particulates into sediments of the harbor and into Massachusetts Bay was apparent in the δ15N values of surface sediments and in sediment profiles. Changes towards more typical marine values over time indicated a lessening of sewage inputs. The incorporation of sewage particulates into blue mussels as revealed by the combination of δ15N and δ34S values in their tissues was also evident and suggested the importance of sewage-derived nutrients to the local food web.
[Show abstract][Hide abstract] ABSTRACT: The decomposition of organic matter and the regeneration of nitrogen in the sediments of Buzzards Bay, Massachusetts were examined by measuring benthic fluxes of oxygen and dissolved inorganic nitrogen (DIN). Benthic respiration (O2 consumption) rates measured from one site yielded an estimate of 6580 g C m-2 oxidized annually. Comparing the annual release of DIN with the consumption of O2 led to an estimate of N loss from the benthic-pelagic system, most likely as N2 gas via denitrification, corresponding to 1432% of the N remineralized from organic matter decomposition. Using path analysis, benthic flux rates of O2 and DIN over a seasonal cycle in Buzzards Bay were determined to be related to water temperature and sediment photosynthetic pigments (chlorophyll a and phaeopigments). The rate of DIN release was also negatively related to the particulate organic N (PON) pool as well. The relationship of benthic fluxes to sedimentary pigment concentrations suggested that pigments were good indicators of labile organic matter input to sediments. Macrofauna appeared to have a direct negative effect, as well as a positive indirect effect on DIN release. Benthic respiration rates were not related to sedimentary particulate organic C (POC) or PON content, or macrofaunal abundances. Release rates of DIN were also unrelated to POC pools. Benthic flux rates measured at 12 sites in Buzzards Bay during August 1989 varied by less than a factor of 2 for benthic respiration and less than a factor of 3 for DIN release. The only environmental factor that emerged from path analysis as related (negatively) to the spatial pattern of benthic flux rates in August was water depth. Other factors, such as organic pools, pigment concentrations, macrofauna, and distance from the New Bedford sewage outfall were not related to the spatial patterns of benthic fluxes in Buzzards Bay. The combination of seasonal and spatial observations indicate that the processes oxidizing organic matter in Buzzards Bay sediments are controlled by temperature and the delivery of labile organic matter to the sediment surface. Benthic flux rates in Buzzards Bay were generally low, but N recycling efficiency was high, relative to similar coastal environments.
Journal of Marine Research 12/1994; 53(1):107-135. · 0.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: To gain insight into the importance of the benthos in carbon and nutrient budgets of Boston Harbor and surrounding bays, we
measured sediment-water exchanges of oxygen, total carbon dioxide (DIC), nitrogen (ammonium, nitrate+nitrite, urea, N2O), silicate, and phosphorus at several stations in different sedimentary environments just prior to and subsequent to cessation
of sewage sludge disposal in the harbor. The ratio of the average annual DIC release to O2 uptake at three primary stations ranged from 0.84 to 1.99. Annual average DIC:DIN flux ratios were consistently greater than
predicted from the Redfield ratio, suggesting substantial losses of mineralized N. The pattern was less clear for P: some
stations showed evidence that the sediments were a sink for P while others appeared to be a net source to the water column
over the study period. In general, temporal and spatial patterns of respiration, nutrient fluxes, and flux ratios were not
consistently related to measures of sediment oxidation-reduction status such as Eh or dissolved sulfide. Sediments from Boston
Harbor metabolize a relatively high percentage (46%) of the organic matter inputs from phytoplankton production and allochthonous
inputs when compared to most estuarine systems. Nutrient regeneration from the benthos is equivalent to 40% of the N, 29%
of the P, and more than 60% of the Si demand of the phytoplankton. However, the role of the benthos in supporting primary
production at the present time may be minor as nutrient inputs from sewage and other sources exceed benthic fluxes of N and
P by 10-fold and Si by 4-fold. Our estimates of denitrification from DIC:DIN fluxes suggests that about 45% of the N mineralized
in the sediments is denitrified, which accounts for about 17% of the N inputs from land.
Estuaries and Coasts 20(2):346-364. · 2.56 Impact Factor