Sampling stations
Baywatchers sampling stations were generally concentrated in the upper half of estuaries and major sub-estuaries to better characterize water quality changes over time. The program has grown over time and expanded to include additional stations within sub-estuaries as well as adding additional sub-estuaries. In 2012, stations were added in Vineyard Sound, which adjoins Buzzards Bay, and in 2017, sampling began at additional stations in the coastal ponds connected to Vineyard Sound.
We assign each station a unique identification code (station ID) and data are integrated into geographic information systems. Station maps are given to monitors (historically using ArcView GIS overlaid on scanned U.S.G.S. quadrangle maps, but more recently, using Google Earth to produce the maps overlaid on aerial images). In a small number of cases, the location of a monitoring site has varied slightly over time—for example, nutrient samples were collected in the Agawam River in a rowboat 250 feet from shore from 1998 to 2007 (station AG2A), but have been collected from a nearby dock since 2008 (station AG2). When a monitoring site’s location has moved, it is given a unique station ID in the database (i.e., AG2 vs AG2A in the example above).
Field sampling
Sampling occurs from late May to September to document conditions when biological activity is highest. Field water sampling is separated into “basic” sampling and “laboratory” sampling days. On all sampling dates, water temperature, salinity, Secchi depth, and total depth are measured in the field. Monitors record these results on hard copy datasheets along with the station ID, sampling date, collection time, name of person sampling, and name of the sub-estuary.
On basic sampling days, D.O. is measured in the early morning (between 6:00 and 9:00 am) to capture typical daily minimum oxygen concentrations before peak daytime photosynthetic oxygen production. Basic sampling occurs on a schedule roughly every five days between late May and mid-September. On laboratory sampling days, oxygen measurements are only made if the monitor has a water quality sonde, as the focus is the collection of samples for laboratory analysis of NH4+, NO3− + NO2−, PO43−, TDN, PON, POC, Chl a, and Pheo. At designated fresh water and low salinity stations, TP and DOC are also measured. Laboratory sampling occurs on four scheduled days each summer (2 in July, 2 in August) during the last three hours of an outgoing tide when concentrations of solutes in estuarine water are expected to most strongly reflect the influence of watershed inputs. While the vast majority of observations were made between late May and mid-September (Fig. 3), some additional basic and laboratory sampling occurred at other times of the year when short-term projects provided opportunities for expanded sampling.
Frequency of Baywatchers samples collected by month between 1992 and 2018. Note difference in y-axis scale for panel c.
The Buzzards Bay Coalition pairs some nearby basic sampling stations with laboratory sampling stations for analysis of sub-estuary water quality. These station pairs have station IDs that end in either the suffix X or N to indicate that they are sampled on basic or laboratory sampling days, respectively.
Basic sampling procedures
Water samples are collected for water temperature, salinity, and D.O. from near the bottom of the water column (0.3 m above the bottom). Where the water column is deeper than 1.2 m, a sample near the surface (0.15 m depth) is also collected to provide information on potential water column stratification. The depth of 0.15 m below the surface prevents entrainment of floating particles and overlying air into the sample bottles. Sampling 0.3 m above the bottom prevents resuspension and capture of bottom sediments by the sampling apparatus.
Water samples for temperature, salinity, and D.O. are collected either with a steel sampling pole or measured in situ with water quality sondes (YSI models 600XL, 600XLM, 6600, EXO2, ProDSS). Sampling poles are 1.5 or 3 m long and marked in 5 to 10 cm depth increments. Sampling poles have 1 L and 0.5 L plastic (HDPE) bottles with rubber stopper closures connected to strings. Poles are lowered to the appropriate depth and then bottles are opened by pulling the strings, first the 0.5 L bottle is opened, followed by the 1 L bottle. D.O. is measured from the 0.5 L bottle, so it is opened first to prevent entrainment of air bubbles into the D.O. sample. Temperature and salinity are measured from the 1 L bottle. Water temperature is measured directly in the 1 L bottle using a thermometer that is calibrated annually. Monitors have primarily used analog thermometers; however, some digital thermometers have been used since 2016. Salinity is measured by then transferring 0.5 L of sample from the 1 L bottle to a 0.5 L graduated cylinder. Specific gravity is measured using a hydrometer that is calibrated annually. Temperature is measured in the graduated cylinder and salinity is determined from a table of specific gravity and temperature.
The 0.5 L bottle has been modified with plastic fittings and tubing at the bottom so that water can be extracted from the bottom of the bottle. Water is siphoned through the tubing to the bottom of a glass-stoppered bottle, overflowing the glass bottle until the 0.5 L bottle is only one-quarter full. Monitors measure D.O. in the glass bottle directly in the field using a modified Winkler titration (Hach Test Kit, Model OX-2P). Briefly, pre-weighed aliquots of manganese sulphate and a lithium hydroxide monohydrate/potassium iodide mixture are added to the sample bottle, which is stoppered and vigorously shaken. The resultant floc is allowed to settle, then reshaken and settled again, before the addition of a pre-weighed aliquot of a sodium phosphate dibasic/sodium sulphate/citric acid mixture. The sample is shaken until this dissolves and the sample is clear, then an aliquot of sample is measured into a separate vial. The sample aliquot is titrated drop-wise using a sodium thiosulphate standard until the sample becomes colorless. The D.O. concentration (mg L−1) is calculated from the number of titration drops.
Secchi depth is recorded by lowering a Secchi disk into water slowly from the shady side of a boat, dock or pier until it just disappears from view. It is then raised and lowered slightly to ensure the proper average depth of disappearance. If the Secchi disk hits the bottom before it disappears, no Secchi depth value is recorded. Total depth is determined when slack is felt in the measuring tape of the Secchi disk.
On basic sampling days, monitors also record the tidal direction (ebb or flood) and the time of the nearest low tide according to the Eldridge Tide and Pilot Book, wave conditions according to the Beaufort scale, and weather status (based on eight potential choices: cloudless, partly cloudy, overcast, fog/haze, drizzle, intermittent rain, rain, snow). Precipitation in the previous 24 hours is noted as either none, light, or heavy. Wind direction is also recorded. All data are recorded on a paper data sheet.
Approximately 20,000 of the temperature, salinity, and D.O. and roughly 5,000 of the pH and Chl a measurements were made in situ using water quality sondes. The majority of these measurements were made since 2000 (YSI models 600XL, 600XLM, 6600, EXO2, ProDSS), though a few were made in the early years of the program (YSI model 51B). The sonde measurements are made following the manufacturers’ standard operating procedures. Instruments are calibrated for temperature, salinity, D.O., pH and Chl a at the beginning of each sampling season. The instrument D.O. calibration is checked prior to each sampling day and re-calibrated in the field if necessary.
Laboratory sampling procedures
On laboratory sampling days, water samples for analysis of dissolved and particulate constituents are primarily collected from near the surface (0.15 m), though a small portion have been collected from near the bottom (0.3 m above bottom). Bottom water samples were generally collected near where there is significant freshwater input that could cause water column stratification. Station ID, water temperature, salinity, Secchi depth, total depth, sample depth, and collection time are recorded in the field on hard copy data sheets. Monitors with sondes also record D.O.
Samples are collected directly into 1 L acid-washed plastic HDPE bottles either by hand or using the sampling pole. All bottles used in water collection were acid washed by the analytical laboratory. Samples bottles are rinsed once with sample prior to filling with the sample. When using the sampling pole, sample bottles are attached and removed using hose clamps.
Monitors filter 60 mL of sample from the 1 L bottle using a 0.2 μm cellulose acetate membrane filter. Filters are first rinsed with 30 mL of sample, which is discarded. A subsequent 30 mL of filtered sample is used to rinse the 60 mL sampling bottle before an additional 60 mL are filtered directly into the 60 mL bottle that was previously acid washed by the analytical laboratory. The remaining unfiltered sample and the 60 mL filtered sample are stored in coolers with ice packs and delivered on the day they are collected to the analytical laboratory. A new membrane filter is used for each sample and the filter holder and syringes are rinsed with tap water after a sample is filtered.
Laboratory analyses
Laboratory analyses were conducted under the supervision of B. Howes at the Woods Hole Oceanographic Institution (1992–1997) and at the University of Massachusetts, Dartmouth (1998–2007), and under H. Ducklow (2008–2012) and C. Neill (2013–2018) at the Marine Biological Laboratory. The methods and instruments described below are those currently used. In some cases, the instruments used have changed, but there has been a significant effort to maintain consistency over the lifetime of the program and to intercalibrate methods/instruments when a change has been made.
Laboratory analyses are designed to accommodate the samples that range from fresh water to nearly full strength seawater and analyte concentrations that range from at or below the detection limit for a method to 1,000 times the detection limit in some cases. Laboratory staff used aliquots of the 60 mL field-filtered sample to perform the dissolved analyses (NH4+, NO3− + NO2−, PO43−, TDN). The filtration for the particulate analyses (Chl a, Pheo, PON, POC) was performed by laboratory staff using water from the 1 L dark sample bottle.
NH4+ is measured colorimetrically by the indophenol-hypochlorite method18. Analyses are conducted in pre-reacted test tubes to reduce blank corrections and absorbance is read on a Cary spectrophotometer with an automatic sipper attachment. NH4+ is analysed on the day samples are collected.
NO3− + NO2− is measured colorimetrically after cadmium reduction19 on a Lachat flow injection analyser (Hach, Loveland, CO). The method was modified for flow injection analysis by Lachat Instruments20. Prior to analysis, samples are refrigerated at 4 °C. Samples are analysed within one week of collection if collected July-August, or if collected outside of July-August, frozen and analysed within 90 days.
PO43− is measured colorimetrically by the molybdenum blue method21 on a Lachat flow injection analyser (Hach, Loveland, CO). The method was modified for flow injection analysis by Lachat Instruments22. Samples are analysed within one week of collection if collected July-August, or if collected outside of July-August, frozen and analysed within 90 days.
TDN is analysed by persulphate digestion23 that oxidizes dissolved nitrogen to NO3− and subsequent analysis of NO3− by colorimetry on a Lachat flow injection analyser. To reduce the magnitude of the reagent blank, persulphate is recrystallized prior to analysis. Samples are stored at 4 °C prior to analysis and are analysed within two weeks of collection if collected July-August, or collected outside of July-August, frozen and analysed within 90 days.
Chl a and Pheo are measured with method of Arar et al.24 by filtering a known volume of water through a 25 mm glass fiber filter (GFF). On the day of sample collection, samples are filtered under low vacuum pressure (<10 psi) in dim light. Filters are then stored in 15 mL centrifuge tubes, in the dark at −20 °C until analysis. Filters are extracted using 7 mL of 90% buffered acetone followed by acidification. Optical filters are used to determine the excitation (420 nm) and emission (670 nm) wavelengths. Extracts are measured on a fluorometer (Turner Designs Model # 10-AU-005-CE). Chl a is calculated from the decrease in fluorescence caused by acidification. Pheo is calculated from the residual fluorescence after accounting for fluorescence by Chl a. The fluorometer is calibrated within one month of the first sampling date using solutions of pure Chl a of known concentrations. A solid secondary standard is used to check the instrument during the summer sampling season. Samples are analysed within two weeks if collected during July-August, or if collected outside of July-August, analysed within five weeks.
PON and POC are analysed by filtering through pre-combusted 25 mm GFF filters, recording filtered volume, drying at 65 °C, and combusting filters in a Perkin-Elmer 2400 or Thermo Flash 2000 elemental analyser25. Sample filters were not fumed with acid to remove any inorganic carbon. Samples are analysed within two weeks if collected during July-August, or if collected outside of July-August, stored dry and analysed within 120 days.
TP is analysed by persulphate digestion of unfiltered water samples following the method of Gales et al.26 and analysis of PO43− in the digest by colorimetry on the Lachat flow injection analyser. Samples are acidified to pH 2 with ultrapure hydrochloric acid (12 N) on the day of collection and analysed within two weeks if collected during July-August, or if collected outside of July-August, frozen and analysed within 90 days.
DOC was analysed by two methods: (1) catalytic oxidation and measurement of the combustion products on a Shimadzu TOC-V (2009 to 2014) and (2) persulphate oxidation on an Aurora 1030 W (2015 to 2018). Samples are acidified to pH <3 with phosphoric acid and stored at 4 °C. During analysis, inorganic carbon is removed by acidification and sparging of the sample with compressed ultra pure nitrogen gas27. Consensus Reference Material was used for sample years 2009 to 2012. In subsequent years, standards were prepared from dried potassium hydrogen phthalate to encompass the range of sample values, about 150 to 2,000 µM C.
Source: Ecology - nature.com
