vbrock@u.washington.edu

In this project, I plan to investigate the origin of suspended matter in Hood Canal, WA, and relate its properties and concentration to dissolved oxygen (DO) concentrations in surrounding water. I will use d ¹³C as a tracer of the relative amount of terrestrial and planktonic organic particles in the suspended matter. d ¹⁵N and C/N ratios will be used as indicators of organic matter source and to determine how much biological utilization of the material has occurred since its intrusion into the canal. These tracers (d ¹⁵N, d ¹³C and C/N) have proven effective due to differences between respective organic-matter pools and relative enrichment of marine-derived relative to terrigenous organic matter (Thornton and Mcmanus, 1994). Oxygen and total suspended matter (TSM) will be measured with an oxygen electrode and a transmissometer attached to a CTD.

A total of four stations, each will have two sets of TSM samples for the analysis of d ¹⁵N, d ¹³C and C/N. One set will be made near the surface (10 m) and one approximately 5 m above the bottom. In addition, continuous TSM and oxygen profiles will be made at each station with a transmissometer and an oxygen sensor attached to a CTD. These data will be used to determine the sources and fates of organic matter in Hood Canal, WA.

Hood Canal is a narrow, highly stratified estuary in the western edge of Puget Sound, WA (Fig. 1). Hood Canal has been described as having sluggish circulation and produces characteristics that are similar to classic fjords (Paulson et al., 1993). Hood Canal is between 2 and 4 km wide throughout most of its length, and is separated from the main basin of Puget Sound by two deep sills (50 and 75 m). In the main basin of Hood Canal, depths reach a maximum of 175 m and tidal currents produce little net advective transport (~2 cm s^-1; Paulson et al., 1993). The main source of freshwater input is at the head of the estuary, and several smaller rivers enter the western edge from the Great Bend to Dabob Bay (Fig. 1) of Hood Canal. Much of the shoreline surrounding Hood Canal is rural in nature and the vacation homes are served by septic tanks. The only National Pollutant Discharge Elimination System (NPDES) is located in Lynch Cove (Fig. 1) and serves a resort inn (Paulson et al., 1993). Levels of dissolved oxygen (DO) have declined since the construction of the Cushman #2 dam in 1930 (Albertson et al. unpublished literature). The distributions of benthic organisms (eg., Azinopsida serricata) reported in the Great Bend of Hood Canal are symptomatic of a community in stress with over half of the organisms being of species that indicate low DO conditions (Llanso, 1998).

The dam delays, by about 6 mo, approximately 40% of the fresh water flowing down the Skokomish River (Jay, 1996). Output from the dam is therefore very significant in yearly averaging of Skokomish River output (Albertson et al., unpublished literature). Albertson et al. (unpublished literature) investigated the effects of stratification caused by the unnatural flow rates of the Cushman #2 dam. By analyzing a three-layer box model, DO levels at depth were found to decrease with an increase in stratification. Periodically, significant exchanges of relatively high oxygenated, oceanic seawater flush out much of the estuary. Such an event did occur in 1998 in Hood Canal, but DO levels quickly dropped back to prior levels (J. Newton, pers. Comm.).

This result suggests that another process besides stratification is playing a significant role in controlling the DO levels in Hood Canal. Eutrophication has been presented as a reason why observed DO levels have declined in Hood Canal. Mackas and Harrison (1997) indicated that the most sensitive areas in Puget Sound to the effects of eutrophication are small tributaries and fjords that have slow flushing rates and that adjoin urbanized shorelines. Most of these regions are in the south and west margins of Puget Sound. Jay and Simenstad (1996) found that the Skokomish River has not suffered major changes in nutrient dynamics and therefore eutrophication is not evident. Jay and Simenstad (1996) also noted that sediment transport, in particular, plays a vital role in linking alterations of fluvial processes with downstream, estuarine consequences. This project will evaluate if the observed chronic hypoxia is a result of altered sediment transport due to altered flows of the Skokomish River.

Thornton and McManus (1994) studied the use of carbon and nitrogen stable isotopes and C/N ratios as source indicators of organic provenance in estuarine systems. d ¹³C was found to be a reliable tracer of organic matter, but C/N ratios and d ¹⁵N were not reliable due to biological fractionation. C/N (Thornton and McManus, 1994) and d ¹⁵N (Owens, 1985) ratios have been shown to record the degree of diagenetic alteration.

Tefry et al. (1994) argued that knowledge of the sources, transport and fates of biogenic carbon onto the Louisiana shelf is critical to understanding development and persistence of bottom hypoxia, as well as to the overall cycling of carbon. A similar study in Hood Canal, WA would prove useful in understanding the development and persistence of bottom hypoxia, as well as the overall cycling of carbon.

Research will be conducted in Hood Canal, Puget Sound, WA, on the R/V Thompson. Samples of total suspended matter (TSM) will be made at four stations. Two stations (HC1 and HC3; Fig.1) will be used as endmembers to which will represent marine and terrestrial environments, respectively. Station HC2 and HC 4 (Fig. 1) will be used as representative values for there respective areas. Each station will have a near-surface (10 m) sample taken and a near-bottom (5 m above bottom) sample taken each time a station is occupied. Samples will be duplicates, with two samples being taken in triplicate to evaluate precision. Approximately one liter of water will be filtered onto Whatman Grade QM-A Quartz filters with a vacuum pump.

TSM will be analyzed for ¹⁵N/¹⁴N, ¹³C/¹²C, C/N, total carbon and total nitrogen using a Finnigan Delta Plus mass spectrometer coupled with a Carlos Erba HC2500 CN analyzer. The mass spectrometer reports isotope data in the form of a bulk ratio, which are converted to the standard d ¹³C and d ¹⁵N forms using equation (1). Due to the differences of d ¹³C in marine and terrestrial sources, d ¹³C will be used as a source indicator of the suspended matter (see Thornton and McManus, 1994; Simenstad and Wissmar, 1985). Thornton and McManus (1994) and Owens (1985) found that d ¹⁵N and C/N ratios record the degree of diagenetic alteration due to respiration. d ¹⁵N has distinct endmember characteristics, but d ¹⁵N values change due to fractionation during organic matter catabolism by bacteria. C/N ratios also have distinct endmember characteristics, but also change as organisms consume the nitrogen and raise the overall C/N ratio. The d ¹⁵N and C/N ratios will be used as evidence of biogenic alterations of the material since its input into the canal. Changes in d ¹⁵N and C/N ratios will evaluate the importance of TSM as a means of oxygen depletion at depth.

All ¹³C/¹²C values will be standardized relative to Pee Dee Beleminite (PDB) standards and ¹⁵N/¹⁴N values will be standardized relative to atmospheric air. ¹³C/¹²C and ¹⁵N/¹⁴N values will be expressed as d ¹³C and d ¹⁵N, respectively, and converted to this notation using the equation:

d ¹³C or d ¹⁵N = ((R_sample - R_standard)/R_standard)*1000 (1)

where R = ¹³C/¹²C or ¹⁵N/¹⁴N. Data quality control throughout the analysis will be ensured by running a reference standard after every 10 runs. This will permit corrections for machine drift to be made prior to calculation of isotope abundance.

Oxygen and TSM concentrations will also be measured in conjunction with every CTD cast. Oxygen will be measured with a YSI O₂ meter and TSM with a transmissometer. YSI O₂ oxygen measurements will be calibrated using Winkler oxygen titrations, as modified by Carpenter (1965). Trefry et al. (1994) found that high concentrations of TSM were responsible for high AOU on the Louisiana Shelf. A possible explanation for the coincident rise in both TSM and AOU is that particles are serving as substrate for bacterial activity as described by Benner et al. (1992) and that net consumption of oxygen is related to the surface area of particles moving along with a given parcel of water. DO calculations and TSM concentrations will be compared to see if there is a correlation between the two. This will evaluate what role TSM is playing in the oxygen depletion of Hood Canal.

Mark Woodworth will be looking at the d ¹⁵N, d ¹³C, C/N, total carbon and total nitrogen in sediment cores taken from Hood Canal to investigate if there are historical signs of eutrophication in Hood Canal preserved in the sediment record. Woodworth has proposed to ²¹⁰ Pb date the sediment cores to obtain the geochronology of the core. The data proposed in this study of TSM on isotope composition should closely correlate with the characteristics of Woodworth’s surface sediments and provide data on the present-day TSM carbon and nitrogen isotopes and C/N ratios.

Project Budget:

Albertson, S.L. & Newton, J. in press.. Investigation of present versus historic circulation

in Hood Canal with a three-layer box model.

Benner, R., G. Chin-Leo, W.S. Gardner, B.J. Eadie, and J. Cotner. 1992. The fates and

effects of riverine and shelf derived DOM on the Mississippi River plume/Gulf

shelf processes. p84-94. In Nutrient Enhanced Coastal Ocean Productivity.

Publication Number TAMU-SG-92-109, Sea Grant Program, Texas A&M

University, Galveston, Texas.

Carpener, J.H. 1965. The Chesapeake Bay Institute technique for the Winkler dissolved

oxygen method. Limnol. Oceanogr., 10, 141-143.

Jay, D.A. & Simenstad, C.A. 1996 Downstream effects of water withdrawal in a small,

high gradient basin: erosion and deposition on the Skokomish River Delta.

Estuaries 19, 501-517.

Llanso, R. 1998 Distribution of benthic communities in Puget Sound 1989-1993. Ecology 98-328.

Mackas, D.L. & Harrison, P.J. 1997 Nitrogeonous nutrient sources and sinks in the Juan

de Fuca Strait/Strait of Georgia/Puget Sound estuarine system: Assessing the

potential for eutrophication. Estuarine, Coastal and Shelf Science 44, 1-21.

Owens, N.J.P. 1985 Variations in the natural aubundance of ¹⁵N in estuarine suspended particulate matter: A specific indicator of biological processing. Estuarine,

Coastal and Shelf Science 20, 505-510.

Paulson, A.J., Curl, H.C., and Feely, R.A. 1993. The biogeochemistry of nutrients and

trace metals in Hood Canal, a Puget Sound fjord. Marine Chemistry 43, 157-173.

Rabalias, N.N., Wiseman, W.J., and Turner, R.E. 1994. Comparisons of continuous

records of near-bottom dissolved oxygen from the hypoxic zone of Louisiana.

Estuaries 17, 850-861.

Simensad, C.A. & Wissmar, R.C. 1985. d ¹³C evidence of the origins and fates of organic

carbon in estuarine and nearshore food webs. Marine Ecology – Progress

Series 22, 141-152.

Trefry, J.H., Simone, M., Nelson, T.A., Trocine, R.P., Eadie, B.J. 1994 Transport of

particulate organic carbon by the Mississippi River and its fate in the Gulf of

Mexico. Estuaries 17 (no. 4), 839-849.

Thornton, S.F. & McManus, J. 1994. Application of organic carbon and nitrogen stable

isotope and C/N ratios as source indicators of organic matter provenance in

estuarine systems: evidence from the Tay Estuary, Scotland. Estuarine, Coastal

and Shelf Science 38, 219-233.

Fig. 1. Hood Canal study area and western WA (inset). Solid dots indicate station locations where samples will be collected.