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Introduction
Recent reports indicate low dissolved oxygen (DO) levels occur periodically in nearshore marine waters along the coast of Oregon and
Washington causing fish and invertebrate mortality. Low oxygen along the seafloor in deeper waters is a normal
event, but not so in nearshore waters of 65m and less. The low oxygen events on the outer coast are caused by
northerly coastal winds moving surface waters offshore in the spring-summer, bringing cold, low oxygen,
nutrient-rich waters towards the coast and to the surface in upwelling events. The nutrient rich surface water
promotes phytoplankton blooms that soon die back, sink and decay, further lowering oxygen levels. The decay of the
phytoplankton lowers oxygen levels until the winds change or cease allowing the water to mix. Although upwelling is
a normal event along our productive coast, several scientists have predicted changes in the oceanography of the
Pacific, which include an acceleration of upwelling events. This acceleration or shorter time frame between
upwelling events could be responsible for the fish and invertebrate mortalities off Washington and Oregon in 2006,
in which depleted oxygen occurred for longer durations and in shallower water.
Abstract
In 2004 the Olympic Coast National Marine Sanctuary (OCNMS) initiated dissolved oxygen (DO) monitoring to determine
the timing, severity, and extent of depleted oxygen levels along the Olympic Coast. In 2004 and 2005, CTD-DO casts
were taken off the RV Tatoosh biweekly between June and early October along 3 cross-shelf transects at Cape Alava,
Teahwhit Head and Cape Elizabeth. In 2006, OCNMS deployed moorings with continuous CT-DO recorders approximately 1
meter off the seafloor along two of these transects and continued casts along the third. No measured oxygen levels
recorded in 2004 or 2005 could be characterized as hypoxic (<2mg/L).
However, in 2004, near hypoxic conditions
(<3mg/L) were routinely measured. In 2005, a few near hypoxic events were measured at >50m depths off Cape
Elizabeth in May-July. In 2006, hypoxic conditions occurred in May and June as far north as Cape Alava and as far
south as Cape Elizabeth. Hypoxic conditions were more extensive at Cape Elizabeth, extending into shallower and
deeper waters and for longer durations. Data co-occurring with fish and crab mortality events reported by the
Quinault Indian Nation in late July near Cape Elizabeth are supported by DO concentrations that got as low as
0.0245 mg/L between 16 and 30 July 2006. No observations of invertebrate or fish mortalities were reported by
observers during the May, June, August, September or October 2006 hypoxic events.
Purpose and Objectives
Since 2001, the OCNMS has periodically observed crab and fish mortalities or received reports from beach walkers,
commercial crabbers and coastal biologists with the coastal tribes, federal and state agencies. Reports of dead
crab and bottom fish littering the shoreline are most common between Kalaloch and the Copalis River, where the
shelf is wide; however, on occasion, reports are received as far north as Makah Bay. In 2004, the sanctuary began
monitoring DO levels to determine the timing, severity, and extent of depleted oxygen levels along the Olympic
Coast.
Methods
In 2004 and 2005, temperature, salinity and DO water column profiles were taken using a Seabird 19 CTD with a
Seabird 23 oxygen sensor approximately biweekly between June and early October along three cross-shelf transects at
Cape Alava, Teahwhit Head and Cape Elizabeth (see map pop up). Stations were positioned at 10m contours between 10 and
100m in depth. These profiles represent point data, only providing profiles at one point in time on a single day.
In 2006, OCNMS installed 4 continuous recorders (Seabird 16Plus units with Seabird 43 oxygen sensors) each along 2
of these transects in order to obtain daily oxygen data throughout the summer season at approximately 1m off the
seafloor. The 100m moorings were abandoned after interactions with fishers and a single recorder installed at one
of the Teahwhit Head stations.
Results/Conclusions
We have defined conditions as hypoxic when DO is below 2mg/L; however, biological stress induced by low oxygen is
seen at levels as high as 4-4.5 mg/L. The Washington Department of Ecology defines DO levels below 3mg/L (about
30% saturation) as "near hypoxic".
· No measured oxygen levels recorded in 2004 or 2005 could be characterized as hypoxic (<2mg/L).
· In 2004, near hypoxic conditions (<3mg/L) were routinely measured near the seafloor in July at the deeper water
sampling sites (>70m) along all three transects, and at Cape Elizabeth at all stations (10m-100m) in June and July
and at deeper stations in September and October.
· In 2005, only a few near hypoxic events were measured - strictly at deeper water sampling stations (>50m) off
Cape Elizabeth during point sampling in May, June and July.
In 2006, with the installation of continuous recorders at 6 to 8 stations from 3 May to 3 October, a more complete
picture was obtained and hypoxic conditions occurred for longer durations, in shallower depths and over a greater
area in 2006.
· In 2006, hypoxic conditions occurred periodically between 31 May and 2 October when the moorings were retrieved
(Tables 1 and 2).
· 2006 hypoxic conditions occurred as far north as our northern-most station at Cape Alava and as far south as our
southern-most station at Cape Elizabeth in the bottom waters at the 42 and 65 meter depth stations.
· At Cape Elizabeth the hypoxic conditions were more extensive, extending from shallow waters (15m station) that
are generally well mixed, to the 100m station throughout the summer months and on into October when the moorings
were retrieved.
· Data co-occurring with the fish and crab mortality events reported by the Quinault Indian Nation in late July
near Cape Elizabeth are supported by DO concentrations as low as 0.0245 mg/L between 16 and 30 July 2006.
· No observations of invertebrate or fish mortalities were reported during the May, June, August, September or
October hypoxic events, which could be due to a variety of reasons, e.g.,
- the events were short in duration or limited in extent;
- DO concentration, though less than 1 mg/L, did not get as low as those measured in July;
- possibly no observers were on the beach to report during the hypoxic time frames;
- the mobile organisms had moved out of the area previously;
- the carcasses were transported elsewhere, etc.
An obvious question is how and what organisms will be affected by low oxygen and what is their tolerance?
To improve our understanding of future hypoxic events and their ecological impacts, OCNMS hopes to expand its
mooring program with additional continuous records and eventually year round moorings. In addition, we continue to
expand our partnerships with British Columbia, Washington and Oregon. With a more extensive monitoring network,
OCNMS and its partners will be able to piece together the picture of what is occurring off the coast.
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