Peter H. Doering

A comparison of methods for the measurement of dissolved organic carbon in natural waters

A small suite of natural samples spanning a wide range of DOC concentrations and salinities were analyzed by three high-temperature combustion (HTC) techniques and persulfate oxidation. One of the HTC techniques, sealed-tube combustion (STC), served as a referee method. Using this method, it was possible to obtain absolute DOC concentrations for the natural samples free of any analytical blank offset. Prior to the comparison of samples, an exchange of carbon-free water and calibration standards showed that all methods were equally well calibrated. Linear correlation analysis was used to differentiate whether the differences observed between methods was due to variable oxidation yields or to an artifact of the instrument blank. Agreement among the various methods was quite good, but yields for all methods decreased compared to the STC technique at concentrations of > 400 μM C. Persulfate results were found to be very similar to HTC results. Finally, carbon-free distilled water prepared by UVH2O2 oxidation or Milli-Q systems had near-zero DOC concentrations and was adequate for blank correction of all the various techniques yielding DOC concentrations in excellent agreement with the referee method.

Effect of the hard clam Mercenaria mercenaria on benthic fluxes of inorganic nutrients and gases

The effect of the hard clam Mercenaria mercenaria (L.) on the exchange of dissolved nutrients (silicate, phosphate, ammonium, nitrate+nitrite) and gases (oxygen, carbon dioxide) across the sediment-water interface was examined in 1983 and 1984 using experimental mesocosms (13 m3), designed to simulate a shallow coastal ecosystem, that allow for reciprocal biogeochemical interactions between water column (5 m) and bottom sediments (∼30 cm deep). Benthic, fluxes, measured during a spring-summer warming period, were compared for mesocosms maintained either with added M. mercenaria (16 per m2, treatment) or without M. mercenaria (control) as a component of the benthic community. Fluxes were within the range observed for the (control) community in situ in mid-Narragansett Bay and exhibited the pattern of increasing flux with increasing temperature observed in nature. For dissolved inorganic (DIN=ammonia+nitrate+nitrite), analytical problems allowed comparison of fluxes only at the higher temperatures (∼17° to 22°C); average DIN fluxes were 57% higher in mesocosms with clams. Fluxes of other nutrients and gases were modelled as exponential functions of temperature (∼9° to 22°C) using a linear regression of the natural logarithm of flux upon temperature in both treatment and control situations. Differences between regression slopes and intercepts for conditions with and without clams were assessed by analysis of covariance. Slopes of the linearized temperature-flux relationships were not significantly different between treatments for any flux measured, suggesting that presence of the clams did not alter the basic relationship of flux and temperature normally observed with this community. For oxygen consumption and silicate production, the regression intercepts were significantly higher in the clam treatment, suggesting that the level of flux was generally elevated in the presence of M. mercenaria by about 20% (oxygen) and 86% (silicate) at any given temperature. Data for carbon dioxide and phosphate showed similar trends to oxygen and silicate, but the variance in fluxes was larger and regression intercepts for the two treatments were not significantly different.

Measurement of dissolved organic carbon by wet chemical oxidation with persulfate: influence of chloride concentration and reagent volume

Abstract The influence of chloride content and reagent volume on the analysis of dissolved organic carbon by wet chemical oxidation with persulfate was evaluated. A strong hyperbolic relationship was found between measured DOC concentration and volume of persulfate added for oxidation in both marine and artificially chlorinated (NaCl) freshwater samples. Freshwater samples showed no such relationship. Precision of measured DOC concentrations averaged 4–7% and did not vary as a function of persulfate volume. Apparently chloride, and possibly other ions, produce significant interference in the oxidation reaction of organic matter by persulfate. Thus, analysis of DOC in seawater, or water with a high chloride content, by persulfate oxidation may yield erroneously low, yet precise, results if asymptotic concentrations of persulfate are not employed. Use of less than asymptotic concentrations of persulfate may account for some of the variability encountered in intercomparison experiments employing persulfate and high temperature catalytic oxidation methods.

Reduction of sea star predation by the burrowing response of the hard clam Mercenaria mercenaria (Mollusca: Bivalvia)

The hard clam Mercenaria mercenaria burrows deeper into the sediment when the predaceous sea star, Asterias forbesi is present. The supposition that this increase in burial depth represents an escape response designed to reduce predation was tested experimentally by regulating clam burial depth through manipulation of the amount of sediment available for burial. Mercenaria maintained at zero depth were eaten by Asterias at greater rates than those held at ordinary burial depths (2.5–3.0 cm). These clams in turn were eaten at greater rates than those maintained at escape depths (4.0–4.5 cm). The results unambiguously establish an anti-predator function for the burrowing response, as well as underscoring the protective function of the fossorial habit. They are not confounded by behavioral predator food preferences, inherent differences between prey species, or debilitating side effects of preventing prey from escaping. Mechanisms by which the burrowing response may reduce predation are discussed and observations on the unreported clam-digging behavior of Asterias forbesi are presented.

Mixing Behavior of Dissolved Organic Carbon and its Potential Biological Significance in the Pawcatuck River Estuary

The distributions of dissolved organic carbon (DOC), phytoplankton biomass (as measured by in vivo fluorescence), total nitrogen and phosphorus, and light extinction were observed on 10 cruises during 1989 and 1990 in the Pawcatuck River estuary located in southern Rhode Island. In the lower estuary, the distance of peak phytoplankton biomass from the head of the estuary was positively correlated with river discharge while the magnitude of the peak increased with decreasing discharge. High light-extinction appeared to limit the accumulation of biomass in the upper estuary. Variability in light extinction was largely (50%) explained by variation in the concentration of DOC. Salinity versus constituent plots suggested that DOC behaved nonconservatively in the estuary. These observational data indicate that the mixing behavior of DOC in the estuary influences light extinction and thus may limit accumulation of phytoplankton biomass in the upper estuary. This interpretation of observational data was supported by experimental work that demonstrated the significant contribution of DOC to light extinction, and by measurements, of phytoplankton productivity that showed greater light limitation in the upper estuary.

Control of Nutrient Concentrations in the Seekonk-Providence River Region of Narragansett Bay, Rhode Island

Six synoptic samplings of nutrient concentrations of the water column and point-source inputs (rivers, sewage treatment plants) were conducted in the Seekonk-Providence River region of Narragansett Bay. Concentrations of nutrients (NH4+, NO2−+NO3−, PO4−3, dissolved silicon, particulate N, particulate C) were predicted using a conservative, two-layer box model in order to assess the relative influence of external inputs and internal processes on observed concentrations. Although most nutrients were clearly affected by processes internal to the system, external input and mixing explained most of the variability in and absolute magnitude of observed concentrations, especially for dissolved constituents. In the bay as a whole, two functionally distinct regions can now be identified: the Seekonk-Providence River, where dissolved nutrient concentrations are externally controlled and lower Narragansett Bay where internal processes regulate the behavior of nutrients. A preliminary nitrogen budget suggests that the Seekonk-Providence River exports some 95% of the nitrogen entering the system via point sources and bottom water from upper Narragansett Bay.

Net system production in coastal waters as a function of eutrophication, seasonality and benthic macrofaunal abundance

Net system production ranged from 13% to 29% of apparent system production in enclosures modelling coastal marine waters. Net production was measured by direct and indirect methods along with factors which impact its magnitude and fate. The direct measures of carbon, phosphorus, and nitrogen content of accumulating flocculent material in enclosures without sediment agreed with indirect measures by net system metabolism and by net sediment storage from nutrient mass balances. Increased nutrient supply, increased the absolute, but not relative, net system production ultimately stored in the sediment. Net production as dry weight of floc did not agree with that calculated from oxygen metabolism owing to a high silicon content of the organic matter. The presence of a benthic macrofauna decreased net system production storage by about 28–54%.

Active enhancement of particle removal from the water column by Tentaculate Benthic Polychaetes

Abstract Two independent experiments were conducted to measure the effects of benthic community structure on the removal rate of 16 µm diameter, radiolabeled particles introduced into unstratified, experimental ecosystems. Particle-removal rates increased in the presence of large numbers of the benthic spionid polychaetes Streblospio benedicti Webster and Polydora ligni Webster. This enhanced particle removal was due to the activities of the worms and not merely to the presence of tube structures projecting above the sea-bed and into the benthic boundary layer.

Reduction of attractiveness to the sea star Asterias forbesi (Desor) by the clam Mercenaria mercenaria (Linnaeus)

Abstract Both in field and laboratory choice tests, the sea star, Asterias forbesi (Desor), was attracted to distant upstream clams, Mercenaria mercenaria (Linnaeus). Clams exposed to upstream sea stars were chosen less frequently by downstream sea stars than clams without sea stars upstream. Sea stars neither attracted nor repelled downstream conspecifics. When clams were exposed to upstream sea stars, their oxygen consumption decreased, as did their pumping rate and activity (as measured by number of visible siphons). The former may result from one or both of the latter. It is concluded that clam and sea star sense each other over a distance by chemical cues. The response of the clam is a general lowering of activity which may result in decreased attractiveness to sea star predators. This response may serve as a defensive measure against distance detection by Asterias forbesi .

Geochemistry of volatile organic compounds in seawater: Mesocosm experiments with 14C-model compounds

Abstract A series of ten radiotracer experiments were conducted in controlled experimental ecosystems (mesocosms) to investigate the behavior of volatile organic compounds (VOC) in seawater. Time-series measurements of the redistribution of 14 C-activity within several major pools—dissolved, particulate, intermediate metabolite, and CO 2 —in the ecosystem made possible an evaluation of the rates of processes—volatilization, biodegradation, Sorption and sedimentation—acting to remove VOC from seawater in summer. The behavior of the model 14 C-VOC fell into three categories. Aromatic hydrocarbons (benzene, toluene, naphthalene) were subject to both volatilization and biodegradation, with mineralization dominating in summer. Chlorinated C 2 -hydrocarbons (tetrachloroethylene) and chlorinated benzenes (chlorobenzene, 1,4-dichlorobenzene, and 1,2,4-trichlorobenzene) were affected only by volatilization and were relatively resistant to biodegradation. Of all the model VOC studied, only aliphatic hydrocarbons (decane and octadecane) were sorbed onto suspended paniculate matter; however, the primary route of loss from the water column appeared to be biodegradation rather than sedimentation. The mesocosm-derived removal rate constants were then applied to estimate summer VOC residence times in Narragansett Bay, Rhode Island of about 1 day for aromatic hydrocarbons, 1 week for the chlorinated VOC and about 1 day for aliphatic hydrocarbons. Residence times in winter might be on the order of 1 week for all VOC.