Harry Hemond

Toxicity of inorganic and methylated arsenic to algal communities from lakes along an arsenic contamination gradient

Abstract The toxicity of arsenate (As(V)), arsenite (As(III)), monomethylarsonic (MMAA) and dimethylarsinic acid (DMAA) to natural algal assemblages from lakes within the Aberjona watershed having different arsenic concentrations was determined by a short-term photosynthesis assay. Total arsenic concentrations in the studied lakes ranged from 1.5×10 −8 to 1.9×10 −7 M. The toxicity of the arsenic species generally decreased in the order of As(V)>As(III)>DMAA for all lakes. Toxicity of As(V) to phytoplankton collected from relatively unpolluted Horn Pond was highest (EC 50 =3×10 −7 M), whereas algae from the polluted Upper Mystic Lake were more tolerant (EC 50 =6×10 −6 M) and those from Halls Brook Storage Area, a highly contaminated lake, were tolerant to As(V) up to 10 −3 M. The sensitivities of the different algal communities to As(III) were similar (EC 50 value=5×10 −5 M). MMAA was as toxic as As(V) in the unpolluted system (Horn Pond). However, photosynthesis of lakes from contaminated lakes was slightly enhanced by MMAA. DMAA concentration in the range of 10 −5 M was strongly enhancing to short-term CO 2 fixation rates of all phytoplankton assemblages; activity increased up to 600% compared to control values. Algae from contaminated sites appear to have adapted to higher As(V) and MMAA concentrations, whereas algae from the unpolluted lake remained sensitive. As(III) represented no chemical stress to the algal assemblages as measured by the assays. The role of DMAA is unclear. Considering the concentrations and the toxicity of As(V) and MMAA in unpolluted systems, one must conclude that As(V) is the major arsenical environmental hazard, however, MMAA might also pose a potential risk in unpolluted systems.

ACCUMULATION AND REDUCTION OF ARSENATE BY THE FRESHWATER GREEN ALGA CHLORELLA SP. (CHLOROPHYTA)

Arsenate accumulation and reduction kinetics at both high and low phosphate concentrations were investigated in the green alga Chlorella sp, isolated from the arsenic‐contaminated Upper Mystic Lake near Boston, MA. Growth rate, accumulated cellular arsenic, and release of As(III) were determined over a range of arsenate concentrations. Arsenate inhibited growth and reduced final cell yield at high phosphate concentration. However, growth rate, final cell yield, and cellular arsenic content were all enhanced by higher arsenate concentrations in cultures grown at a low concentration of phosphate. The traditional view that phosphate‐limited cells are necessarily more sensitive to As(V) toxicity may not be correct. The reduction rates of As(V) by Chlorella sp. obtained in our laboratory were similar to net reduction rates measured in epilimnetic water from the Upper Mystic Lake, demonstrating the importance of phytoplankton in arsenic reduction in freshwater.

NEREUS: engineering concept for an underwater mass spectrometer

NEREUS (novel, efficient, rapid evaluation of underwater spectra) is a self-contained underwater mass-spectrometer system capable of continuous measurements of gases and vapors dissolved in the water column. Rapid in-situ measurement minimizes or eliminates artefacts that are often a problem when collecting and storing samples for analysis of volatile constituents, and provides higher spatial and temporal resolution of chemical patterns than is possible with conventional techniques. NEREUS is designed to operate either independently on a mooring or winch line, or aboard an autonomous underwater vehicle (AUV), such as the Sea Grant Odyssey class submersible. Underwater mass spectrometers such as NEREUS have many important applications to practical problems, such as pollution monitoring, as well as to fundamental Earth systems research.

Field Testing of Lake Water Chemistry with a Portable and an AUV-based Mass Spectrometer

Two mass spectrometers (MS) are tested for the measurement of volatile substances, such as hydrocarbons and metabolic gases, in natural waters. KOALA is a backpackable MS operated from above the water surface, in which samples are pumped through a flow cell using a syringe. NEREUS is an underwater instrument hosted by an autonomous underwater vehicle (AUV) that is linked to a communications network to provide chemical data in real time. The mass analyzers of the two MS are nearly identical cycloids, and both use flat-plate membrane inlets. Testing took place in an eutrophic, thermally stratified lake exhibiting steep chemical gradients and significant levels of methane. KOALA provided rapid multispecies analysis of dissolved gases, with a detection limit for methane of 0. 1 ppm (readily extendable to 0. 01 ppm) and savings of time of at least a factor of 10 compared to that of conventional analysis. The AUV-mounted NEREUS additionally provided rapid spatial coverage and the capability of performing chemical surveys autonomously. Tests demonstrated the need for temperature control of a membrane inlet when steep thermal gradients are present in a water body, as well as the benefits of co-locating all sensors on the AUV to avoid interference from chemically different waters entering and draining from the free-flooding outer hull. The ability to measure dissolved volatiles provided by MS offers potential for complementarity with ionic sensors in the study of natural waters, such as in the case of the carbonate system.

An Infiltrometer to Measure Seepage in Salt Marsh Soils

This study considers the design, construction, and operation of a device for measuring how much water seeps across the surface of salt marsh soil as it is inundated by flooding tides. The device was used in Great Sippiwisset Marsh, Cape Cod. Preliminary results confirm that infiltration during flood tide is followed by widespread exfiltration, of comparable magnitude, during ebb. Total water flux decreased with increasing distance from adjacent creeks.