Environmental Monitors on Lobster Traps
Final Report on Phase II: Salinity

This is an old unfinished DRAFT last modified 19 August 2004
where the most-up-to-date version is posted elsewhere.

Abstract

In order to demonstrate the concept of local lobstermen contributing to our ocean observing systems,  we had proposed,   in "Phase II" of the Environmental Monitors on Lobster Traps Project, to have them secure Seabird Microcats to their gear.  The experiment was carried out as expected.  Several lobstermen collected multi-month time series of salinity and temperature at fixed locations.   A total of 66 months of hourly data were collected  in depths ranging from 55 to 210 meters.  Average salinities ranged from near 32 to over 35 PSU. While many of the salinity records  are uncertain due to potential fouling of the conductivity cell by  fine-grain bottom sediment,  the collaboration was generally successful.  As found in all phases of the project, the New England Lobstermen are willing and able to assist in deploying oceanographic instrumentation.

The methods used to conduct the operations are describe here in full including both the protocol we used as well as alternative protocol we would suggest for such an endeavor in future. The preliminary results are presented  in tabular and graphical form along with in-depth analysis.  Discussion of particular events at  all monitoring sites and comparisons with those from nearby Gulf of Maine Ocean Observing System moorings are included as well.  Salinity is plotted along with concurrent measurements of temperature, wind, and river discharge.

Background

Given recent findings of source waters entering our region from the north,  there is an obvious need to assess the influx of the fresher (low salinity) water mass as it is transported into and around the Gulf of Maine. Is there a detectable increase in the Canadian ice melt waters?  Will climate change have a significant effect on the conditions of our coastal waters?  For purposes of monitoring the influences of  advective water masses, salinity is a far more effective tracer.  Hence, as a natural extension of  our Phase I temperature probe project funded in year 2000, we had proposed phase II: salinity.

Evidence of remote source waters affecting our waters have been published (Loder et al., 2001;  Smith et al., 2001; and Houghton and Fairbanks, 2001).  These papers provide an indication of low-salinity episodes transported from north to south.  Periods of low-salinity appear for several months at a time and can be tracked at several locations along the coast (Mountain and Taylor, 1998).  The objective here was to extend this idea to include several sites within the Gulf of Maine.  If this advective hypothesis holds true, empirical data alone may then help in forecasting the arrival of these anomalous events at downstream locations.  The effect of local river runoff also plays an important role in the interannual variability of salinity at many locations off the coast of Maine  (Mountain and Manning, 1994) and  the inner Mid-Atlantic Bight (Manning, 1991).   The challenge remains however in differentiating these advective influences from the heating/mixing processes that take place locally.  Studies have found a near-equal contribution from each of these processes  (Mountain and Jessen, 1987) .

The very interesting possibility of submarine freshwater discharges affecting the bottom water conditions around the gulf and, in particular,  along the bathymetically complicated coast of Maine has recently been proposed.  This phenomenon was not even considered as a potential influence to near-bottom salinity levels prior to the start of this project.  While these point source inputs of fresh water do not play a significant part in the overall variability of the gulf's salt budget,  the possibility that they exist  can not be ignored.  This new hypothesis that nutrient-rich freshwater is  injected  into the near-bottom marine environment in certain geological formations along the coast of Maine is certainly intriguing and worth further investigation.

Methods

On 24 April 2002,  lobstermen Nick Lemieux, Jim Tripp, and Stevie Robbins III met in Portland Maine for training. The others, Marc Palombo, Steve Keane, and David Johnson, were trained separately.  The following topics were covered over the course of a few hours:

A multi-page  web-served document  "User Manual  for eMOLT Participants Deploying the SEABIRD SBE37sm  Microcat Temperature and Salinity Probe" was provided to each participant with complete instructions on  the entire process.     Sections in this manual include every thing from setting up the instrument to downloading data.  The final chapter listed in detail "Who's responsible for what".

In addition to a Microcat  (Figure 2),  each lobstermen was equipped with a Niskin bottle and several glass sample bottles.  The apparatus is used to capture samples of seawater at selected depths in the water column.  The Lamotte units (see Figure 3) we purchase for $199/each are  a 1-liter sampler of clear acrylic furnished with a 20 meter calibrated line and a lead collar which assures rapid descent and minimal drift. A brass messenger triggers a release mechanism to seal  the sample chamber with two fitted rubber plungers at the desired sampling depth. The built-in side outlet and flexible tube allows for  removal of the water sample.  Most participants experimented with this instrument but, as discussed below,  only a few actually made use of the unit as it was proposed.  Ideally,  calibration samples are required along with any measure of salinity in order to calibrate the instrumentation and correct for offset/bias.

Seabird Microcats, setup to record hourly samples, were secured directly to traps.   As pictured in Figure 3,  the instrument was mounted horizontally in the upper section of the trap so that it rested a foot or more above the seabed.  While each lobstermen secured the instrument in a slightly different fashion, the basic configuration was the same.  The instruments were deployed for multiple months (see Table 1)and often hauled during normal fishing operations.  Occasional hauls allowed the inspection and a deck hose rinsing of conductivity cells. In some cases,  the instrument remained on the bottom for the entire deployment.  Instruments were deployed in a variety of bottom habitats from sandy bottom (highly energetic tidal flow) to deep muddy (relatively stagnant) environments.

Figure 3.  Lamotte water sampler used for Microcat calibration samples.


Figure 4.  Microcat salinity probe secured on the underside of a lobster trap (photo by Norbert Lemeiux).
 
 
 
 
 

Whenever it was convenient (such as prior to fish forums or lobstermen association meetings where both industry & science partners would be attending), the probes were hauled-in, detached, and brought to shore.   This occurred at least a few times for each participant except in one case where only one continuous series was collected.   The data was downloaded each deployment and processed with a series of software routines:

The data is now archived in NOAA/NEFSC ORACLE tables. The site information  is stored in "emolt_site" while the data is stored in "emolt_sensor".  While some haul information is stored in a "emolt_set" table, it is a limited collection and does not apply to the moorings that include salinity.  No attempts were made to correlate salinity variability with haul counts.

Data was posted on the emolt.org website within a few days of downloading.  Plots were generated and posted on the individual's website under the "Results from the Field" site as well as the "What's New" site.   Data was served through the Distributed Oceanographic Data System (DODS) and accessible by the Gulf of Maine Ocean Observing System.   GoMOOS provided a internet mapping service where users can click on zones in order to select particular eMOLT mooring datasets, see their approximate positions relative to other moorings, and view time series plots.
 
 

Findings

The basic mooring information is listed in Table 1 below.  More than 66 months of data were collected  in depths ranging from 55 to 210 meters (See Figure 5).

The range of values observed at each site were within those expected from historical records but, again,  the margin of error is difficult to quantify given the effects of real oceanographic events being so similar to those effects potentially due to cell contamination (see discussion below).   Figure 5 is presented in order to depict the relative time periods each sensor was deployed.  Initial test were conducted in Woods Hole (WH02) starting in late 2001, most sensors were first installed by late 2002 and the few of the sensors were last recovered in early 2004.  Note the consistent salinity scale is posted in the lowermost panel, ranging from 32 to 35 PSU.  Except for Marc Palombo's salty record (>35 PSU, 2nd panel from the top) from the Hydrographer Canyon on the shelf edge and Dave Johnson's fresher record (<32, bottomost panel) near the river outfalls, most fall within the range of 33 and 34.  In order to depict more details associated with these series each is plotted individually in  Figure X  along with the associated wind and temperature records.  Results from each probe location is presented separately below.  Each record is compared to the Microcat records on nearby GoMOOS moorings. GoMOOS mooring sites are denoted by the yellow dots in Figure 1.
 

Table 1.
 
Site Region Lobstermen Latitude Longitude Depth(ftm) Depth(m) #months** Association Ave Std Min Max
TA15 Hydrographer's Canyon Marc Palombo 4003.75 6904.00 115 210 12.1 AOLA 35.37 .12 34.98 35.86
NL01 Downeast  Norbert Lemieux 4438.55 6702.03 50 91 14.7 DELA 32.29 .45 31.18 33.15
DJ02 Casco Bay David Johnson 4338.84 7008.57 22 40 3.5 MeLA 31.77 .75 29.18 32.74
SK01 Mass Bay Steve Keane 4306.00 7026.00 30 55 5.7 MaLA 32.30 .14 31.81 32.64
RS01 mid-coast shallow Stevie Robbins III 4401.00  6832.80 39 71 14.8 DELA 32.54 .20 32.06 33.18
JT04 mid-coast deep Jim Tripp 4346.30 6840.20 70 128 8.4 MeLA 32.46 .28 31.83 33.40
WHAQ* Woods Hole Jim Manning 4132.50 7040.30 1 2 7.3 NOAA 32.08 .21 30.47 32.49
Total 66.5

* test site
** time series collected at hourly rates

Hydrographer's Canyon

The first  lobstermen to put a Microcat on his trap was Marc Palombo.  Marc was the first and probably the most active participant in the eMOLT project in general with temperature series collected nearly a decade ago.  He has a total of 139 well-documented deployments.  He fishes in a very dynamic area affected by a combination of shelfedge processes.  While his temperature series will be presented in the final report of the temperature project, a brief summary and anlysis of his salinity records are as follows.  As depicted in the 7-day running average salinity in Figure X,  typical salinities in this region are greater than 35 PSU but are significantly modulated by tides, winds, and especially the offshore influence of Gulf Stream ring passages.  The event om Oct 2002 was susequently observed at other sites located to the southwest of Palombo's and, as indicated in Figure X, propagated to the Mid-Atlantic Bight in November and December.  It is difficult to make estimates of ring propagation speeds from these records since what is often observed at any site is often not the ring itself but a perturbation of the shelfslope front on the leading edge of the ring.  As depicted in Figure X, for example, locations marked by black dots are affected by episodic "streamers" or eddies spawned from eddies. The shelf-slope front is often detectable at these deep sites in the form of temperature and salinity oscillations.  Some of the temperature sites document tidal variations of several degrees.  Figure X depicts a moderate case of a few degrees and a few tenths of a PSU. We can determine from this figure, for example, the front was in the vicinity of this probe for a few days centered around 07/08, is advected away, and then reappears on 07/14.  In other words, the presence or absence of the front at anyone location can be determined by the tidal variability of both temperature and salinity.

Mass Bay

Steve Keane's deployments in Mass Bay provided data through the summer (Figure Xa) and fall (Figure Xb) of 2002.   As seen at the 50m Microcat on the upstream GoMOOS mooring "A",  SK01 recorded a  gradual freshening in May  through most of June.  In late June, these deep sensors were apparently capped off from the surface waters at stratification took effect.  Both T & S held fairly steady through July with small intermittent variations possibly due to oscillations in the wind (top panel) before a gradual increase in August and September.  Steve obtained two water samples during this first deployment on 27 June and 10 October.  There is a large descrepancy in the first sample (~0.3PSU) that is likely due tmay have been due to a number of factors. (see discussion below).  A most interesting pair of events occurred during the fall deployment (Figure Xb) on the 7th and 17th of November.  Drops in salinity of nearly 1/2 PSU occurred suddenly and held steady of a few days.  Given that there was not a concurrent change in temperature, these events are a prime example of potential fouling by small grains of mud or sand in the conductivity cell.  While there is the possibility of downwelling events causing the relatively fresh coastal waters of Plymouth to be forced to the deep, one expects a corresponding rise in the temperature signal that clearly did not occur.

Casco Bay

Dave Johnson was the last participant to deploy a probe.  His short record is perhaps the most interesting but the most difficult to explain. His was obtained in a very dynamic, almost-estuarine, environment just outside Casco Bay.  As discussed in more detail below,  first looks at the erratic data concluded possible contamination of the conductivity cell but further examination and comparison with nearby GoMOOS records suggest there is at least some chance that the episodic events may be real phenomenon associated with river plume dynamics.  Visual inspection of the Kennebec River discharge (data downloaded from USGS, the Forks guage),  depicts a few small events during the fall but no consistent coherence with salinity at either the GoMOOS mooring "C0204" or the eMOLT site "DJ02".  The sudden drops in the former in December could potentially be the result of the relative large discharge earlier that month.

Deep Mid-Coast

Jim Tripp fishes the deep waters beyond Matinicus Island.  His is the most suspect of all records. The decline in salinity in August 2002 (relative to the 50m record at GoMOOS buoy E) and the abrupt increase of 1.5 PSU in December are probably due to sediment particles or biofouling of the conductivity cell followed by a flushing. 
 
 

Shallow Mid-Coast

Stevie Robbins III, out of Stonington Me,  obtained  a short record in the summer of 2002 (see "RS01" line in Figure X) and then one of the longest time series for most of 2003.  When plotted against GoMOOS Buoy I (Figure X),  the salinity record again is  often nearly a full PSU less than the 50m record from the GoMOOS mooring.  While there we might expect a variation in salinity as the core of the WMCC passes overhead, the temperature record should depict a change as well.  We are again left with doubt as to the validity of these values.  The abrupt increases in salinity during the fall 2003 are more likely due to flushing of the conductivity cell by either natural currents or participant hauling.  Unfortunately,  a detailed log of exactly when the instrument was hauled is not always available.

Downeast

Norbert Lemieux and his son Nick also provided a long series.  Since its such a long series, 2002 and 2003 are plotted separately in Figure X and Xb, respectively.  The first year is plotted against the records from mooring "I" downstream.  According to preliminary results of the eMOLT drifter project, the typical transet times between NL01 and mooring I is little more than a week.  While we can expect  the conditions at the two sites to be exactly the same due to the differences in the depths of sensors and the distances from river plumes, we see that they are very similar nevertheless. The temperature is the same within a few degrees and the salinity  is slightly saltier at mooring I downstream.  While the temperature peaks in late September the salinity peaked later in October.  Plotted against GoMOOS mooring "J" during mid-2003 (Figure Xb), the record indicates a slightly cooler and saltier conditions off the coast of Cutler relative to that measure at 10m  near shore.  There is a chance that the two slight depressions in salinity on spring 2003 at "NL01" may have been the result of the two large drops in salinity as measured about a week previous at J but without other mooring data around to substantiate this possibility, we are left to the uncertainty again in the lobster trap readings.

Woods Hole

Preliminary instrument test were conducted in Woods Hole during in 2001 and 2002 before any were distributed to lobstermen.  The initial test were discouraging due to problem with fouling in Woods Hole Harbor.  As previously discovered by others (Maureen Taylor, personal communication),  hanging a conductivity sensor off the dock in the inner harbor results in contaminated data probably due to the prevalence of motor oil and other effluents.  This was not only a problem with Seabird Microcats both other instrumentation such as the YSI model deployed simutaneously (Figure X).  After these initial test, the instrumnet was deployed in the NEFSC Aquarium tanks for a lengthy examination.  As noted in figure X ,  water samples were taken on a near-weekly basis to test the accuracy of the electronic sample.  Satisfactory results were obstained.  It is interesting to note the gradual increase of salinity documented in early Winter.  The rapid drop in mid-January was likely due to the aquarium personnel deciding to dump a load of warm water from the fresh water tap into the tank.  After these intial test the instrument was deployed for nearly a year which resulted  in what may be the longest hourly salinity record from Woods Hole Harbor (see Figure X).
This seasonal cycle during 2001 and 2002 is less obvious in the subsequent months of 2003 when only an occassional Niskin sample was obtained off the dock (Figure X).  A total of 79 bottle samples were taken on time periods ranging from a week to a month by submerging a Niskin bottle mid-way through the water column (being careful not to distrurb the bottom sediment) and releasing a messenger to trip the mechanism.  This bottle data  extended the electronic record noted in Figure X through most of 2003.  In several cases, samples were taken both off the dock and inside the aquarium where harbor water is pumped.
The differences bewteen the two sets were not consistent and likely less than the error associated with the method of comparison.

In order to investigate the longterm character of salinity in Woods Hole Harbor,  historical data from Woods Hole Lightship was obtained from Kathy Elder (WHOI) and plotted along with the salinity data collected at the NEFSC Milford Aquarium over a period of a few decades (Figure X).  The seasonal cycles and anomalies are given in Figure Xb and Xc. It is clear from Figure Xc that the Milford site is a brackish estuarine environment typically at around 25 PSU with minimum salinities as expected in April.
and maximum in late October. Both these datasets were entered into the eMOLT database along with many other longterm temperature series at various state, gov't, and commercial  institutions.  The draft write on this ancilliary project is posted at whwt/newt.html.

Discussion

One of the most serious failings of the project was  in obtaining adequate water samples.  The original objective of obtaining near-monthly calibration samples with a Niskin bottle turned out to be unrealistic for a variety of reasons.  The unit we purchased was adequate for the operation except that, in some cases, the lead line was not long enough.  In these cases it was necessary for the participant to add line to the tether.  This was especially a problem in the case of Palombo's where the water depth exceeds a few hundred meters where not only is it physically difficult to deploy the unit but the time involved with such an operation makes it economically unfeasible.  In  this case and especially in the case of Norbert Lemieux's, the tidal velocity was so extreme that bottle was not weighted sufficiently to easily get to the bottom.   In cases of muddy bottom (Johnson and Keane),  the bottle samples were too easily fouled due to sediment resuspension.  The difficulty  of getting a "near-bottom" sample without disturbing the sediment was not considered  in the protocol until it was too late.  While the lead line was incrementally marked,  attempts in the future will need to have a well-marked maximum extension that is appropriate for the particular site at a particular slack tide.

If lobstermen are to obtain water samples,  alternative protocols need to be developed.  The protocol  could be radically modified, for example, to take "near-surface" samples instead.  This would require a conscious effort on the part of the participants.  In this scenario, the participant would take water samples on an opportunistic schedule whenever they happen to be hauling a sensor in calm seas and  when the instrument was scheduled for its hourly sample.  The salinity sensor would need to be hauled up just below the surface (preferably in view) while the Niskin is deployed as close as possible in time with the scheduled electronic sample.  Another alternative  that would allow for undisturbed near-bottom samples would be to deploy the trap (with the salinity sensor installed) along with a traditional Niskin bottle permanently attached to taunt mooring line a meter or two above the trap.  This "taunt mooring line" would be distinct from the normal mooring line and have its own flotation but would only be recoverable in low tide situations.   The lobstermen could then take advantage of low tide situations during a scheduled electronic sample by dropping a messenger down the taunt line to trip the Niskin bottle.  The trawl is hauled, the sample is drawn, the Niskin bottom is reset, and the entire mooring is redeployed.

It was necessary to look closely at potential contamination of  conductivity cells by fine grain sediments or fouling. Initial look at the most uncertain of all the records, that from site DJ02,  indicated it may be fouled.   This preliminary conclusion was based on the fact that a) the record was highly variable,  b) the temperature did not seem to vary along with the salinity,  c) unlike other sites, the instrument was not hauled during routine fishing operations (hauling would tend to provide occasional flushing of the cell) and  d) the lobsterman reported the site as "muddy".  A closer look however indicates that , given the location of the probe at the mouth of Casco Bay in relative shallow water,  the "highly variable" time series may have resulted from real estuarine processes.  After looking at the near-surface salinity records of the nearby GoMOOS Buoy C,  the type of variation observed at DJ02 does not seem that implausible.  In that location, for example, drops of nearly 2 PSU were observed to occur, for example, in mid-November over the course of a few hours. While the episodic events at DJ02 are not coherent with those at Buoy C  (they are located in very different regions of Casco Bay),  the degree and frequency at which they occur, are similar (see the bottommost panel of Figure X).

The episodic variability at the other eMOLT sites were not nearly as dramatic as at DJ02.  In the Fall of 2002, for example, the variations at three eMOLT sites were similar in magnitude to those observed at the GoMOOS side "I" (see Figure X) except that the tidal variations are apparently more pronounced near the bottom.  Note that changes in the near-bottom conditions are likely to be more abrupt than the changes that occur in the upper water column due to the structure of the near-bottom front being more vertical.  While the halocline lies relatively horizontal in the water column,  the orientation of the isohalines is altered by the bottom-boundary layer to be vertical.  This is often referred to as the "foot-of-the-front". This mechanism  is especially plausible the case of Stevie Robbins' case (71m) where the probe may be  located nearby a persistent tidal front. It appears that the sensor was exposed to some edge of the frontal gradients throughout the series.  Is it possible that the axis of the front was perturbed, for example,  on 08/20 (and again on 09/10) where the probe is then exposed to the opposite side of the front?  Notice that these abrupt changes occurred when the  tidal variation was greatest.   Again, without adequate calibration samples at critical times in the record, the question remains unanswered.  In hindsight,  a pair of instruments deployed by each lobstermen either in the same location or is slightly different depths would help resolve the problem. Having instruments in depths differing by ~5 meters, say, would help determine the speed of front translation.

It should be noted that none of the sensors were visibly fouled on recovery.  Participants were trained to visually  inspect the condition of the cell and note any obvious fouling.
Nothing was noted. The units were all returned in good conditions with conductivity cells clean and free of particles.  While the cell may appear free of particles (Figure XX), even the slightest film or temporary alteration of the geometry can apparently bias the conductivity reading.  The cells may have been partially block while moored and then flushed during the hauling operation.


 
 

In order to come to a better understanding of this potential-fouling problem,  an experiment is underway at the time of this writing in the northeast portion of Casco Bay.
The marine science studies of Dr. Ed Laine and his students at Bowdoin College have led to an investigation of near-bottom salinity.  After several class trips where Conductivity, Temperature, and Depth (CTDs) casts observed lower-then-expected levels of near-bottom salinity near particular sites in this area of "Quohog Bay".  Hearing this curiousity led Manning to visit Laine during the fall of 2003 at Bowdoin.  After some discussion and subsequent visits, a plan was devise to deploy the eMOLT microcats at this location to test a) the problem of sediment interference and b) potential existence of submarine freshwater discharges.  After the Microcats were returned to Seabird for cleaning, calibration, and replatinizing, two of them were mounted on an old CTD Rosette cage (obtained from the WHOI surplus).  One is mounted to rest 20cm above the bottom (similar to a lobster trap mounting) and the other is 1 meter above the bottom. The units were securely fitted to the cage with the help of the Bowdoin machine shop and deployed in approximately 10 meters of water with the help of MER associates on 22 July 2004.  The recovery is planned for September 2004. While eMOLT funded the instrument refurbishment,  Bowdoin (Laine et al ) funded the new batteries, rigging, and deployment.  Bowdoin has also conducted a series near-weekly of CTD cast for calibration purposes.  Being a geologist,  Laine is interested in the possibility of submarine discharges occuring at particular geological formations.  If the discharge is confirmed at this location,  investigations at other similar structures will  likely be made in the future.


 

As noted in the previous paragraph,  all  Microcats were shipped to Seabird (except for the one lost) to get refurbished after eMOLT deployments.  All units were calibrated "as received",  cleaned, replatinized, and calibrated again  (In Steve Keane's case, unit 130, replatinizing was not necessary). The results of this operation is documented in the Table below.  Salinity drifts of this order are not insignificant.   Variations of nearly 0.005 PSU per month are small (approximately 1/10th the natural variation due to seasonal change in this area) but values of ~0.05 per year close to the magnitude associated with interannual changes.  The drift Seabird notes for these instruments is in the same order of magnitude at those of other instruments we have deployed in the past.  Instruments that are typically turned-around on a bi-annual basis have drifts on the order of 0.001 PSU/mth.  These values are often dependent on the depth of the instrument in the water column with those in the deep being less fouled.  Nevertheless, if this experiment is planned again in the future, it will imperative to schedule a  recalibration and cleaning of instruments more frequently than annual rate.  The advantage of lobstermen-deployed moorings is the regular opportunity to inspect and clean/flush the instrument.  An accurate log of cleaning operations need to be kept. Since the focus of this pilot project was on shorter term variations and the fouling appeared to be intermittant,  no corrections were applied to the archived data.

Table X.  Results of "as received" calibration.
Site Serial# Drift in salinity (PSU/mth) PSU/year Drift in temperature (degC/year)
TA15 126 -0.0039 0.0468 +0.00017
RS01 127 -0.0041 0.0492 +0.00072
NL01 128 -0.0034 0.0408 +0.00064
JT04 129 not recovered
SK01 130 -0.0008 0.0096 +0.00004
DJ02 935 -0.0027 0.0324 +0.00024

 

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