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Signature Datasets

The Signature Datasets of NGA LTER distinctively illustrate the ecological processes that occur in the Gulf of Alaska. We’ve chosen to present them mainly as time series aggregated over the complex horizontal or vertical structure that is typical of our area. Therefore, they are simpler to understand and to use than the original datasets from which they are derived.

These datasets are currently under development.

Dataset Description

Each Signature Dataset consists of an ASCII data file, a plot of the data, and citations of the archives that contain the original data. Sometimes these citations refer to several different archives.  This is because the multi-decadal data collected in the Gulf of Alaska span several different funded projects and numerous researchers. For example, the original data might have formats that differ according to who prepared them. Additionally, data archives are often split among separate institutions. All these archives can be found separately in our data catalog.  However, the Signature Datasets pull them together into products that are immediately useful for synthesis projects and numerical model verification.

Wet-weight biomass of zooplankton in May and September, 1997-2020.

This dataset contains the wet-weight biomass in grams per cubic meter of the major crustaceans in the NGA. It highlights the importance of large copepod species during spring-time, which is a fundamental element of the NGA ecosystem.  Values are vertically averaged over the upper 100 m for each early May and mid-September (or early October) from fall 1997 to present. Horizontally, they are averaged across the Seward Line stations – typically 13 stations stretching nearly 250 kilometers from the mouth of Resurrection Bay to well past the shelf break. 

Values for large copepods (species where adults have prosome length >2.5 mm) and euphausiids are taken from opening/closing nets of 0.5 mm mesh collected at night, while small copepod data comes from smaller nets of 0.15 mm mesh collected during daytime. Data for the last 1-2 years is typically provisional because it is based on fewer than the full number of stations sampled.

Data Access

Signature Data File

Derived from these Archives

Mixed Layer Depth (MLD) averaged over different portions of the Seward Line (top)
and mid-shelf salinity (bottom) in May, 1997-2020.

Records in this Signature Dataset illustrate physical conditions along the Seward Line from 1997 to 2020. The original measurements were made at stations from GAK1 to GAK13 during multiple cruises each year. Parameters Salinity and Temperature describe the physical environment. In addition, calculated parameters Density and Brunt-Vaisala Frequency can be used to identify water masses and gain insight into physical processes such as vertical mixing.

In order to summarize the original datasets in a form that can be used in multiple situations, data have been averaged by depth and cross-shelf station location (station clusters). The Mixed Layer Depth (MLD) measures the thickness of the well-mixed portion at the surface; data have been averaged over this depth because of its importance to biological processes. The sampling times have been simplified by grouping data together by the month when the cruise occurred.

Data Column Contents:    

  1.  Year
  2.  Month
  3.  Cluster GAK# Station Start
  4.  Cluster GAK# Station End
  5.  Depth Layer Start (m): depth of shallowest measurement. If -1, then averaging is from surface to MLD.
  6.  Depth Layer End (m): depth of deepest measurement. Might be MLD.
  7.  Temperature (degrees Celsius)
  8.  Salinity (nondimensional)
  9.  Density (Sigma-t, kg/m^3 – 1000)
  10.  Brunt-Vaisala Frequency (N^2, 1/s)

Station Clusters descriptions:
  Coastal: GAK1 and GAK2
  Mid-shelf: GAK5 and GAK6
  Shelf break: GAK8 and GAK9
  Slope: GAK12 and GAK13

Depth Layer definitions:
  Surface: 0 – 10 m
  Mixed Layer Depth: 0 m to MLD (indicated by -1 in the Depth Layer Start column)
  Deep: Either 50 m above the bottom to bottom, or 250 m – 300 m if bottom depth > 300 m

Data Access

Signature Data File

Derived from these Archives

Time series of Temperature (top) and Salinity (bottom) anomalies
at the above 50 m GAK1 mooring location, 1970-2020.

This dataset consists of temperature and salinity measured at the location of the GAK1 mooring in the Northern Gulf of Alaska (59° 50.7′ N, 149° 28.0′ W). Sampling began in December 1970 with reversing thermometers and bottle samples at standard depths several times a year. In time, the amount of sampling increased, and continues today with monthly CTD profiles and a year-round instrumented mooring. The mooring deploys data loggers at 6-7 depths ranging from just below the surface (20-30 m depth) to close to the seafloor (250 m depth).

In order to combine measurements at these increasing temporal resolutions, data have been averaged to monthly means. Additionally, to account for different vertical resolution, data are averaged in near-surface (0 – 50 m) and near-seafloor (150 – 200 m). The resulting dataset is a “best combined” aggregation of ship-based CTD profiles and mooring-based CTD data logger records over five decades.  Anomalies relative to the mean of these time series are also included.

Data Column Contents:    

  1. Year  
  2. Month
  3. Surface Temperature (degrees Celsius)
  4. Deep Temperature (degrees Celsius)
  5. Surface Salinity (nondimensional)
  6. Deep Salinity (nondimensional)
  7. Surface Temperature Anomaly (degrees Celsius)
  8. Deep Temperature Anomaly (degrees Celsius)  
  9. Surface Salinity Anomaly (nondimensional)
  10. Deep Salinity Anomaly (nondimensional)

Depth Layer definitions:
Surface: 0 – 50 m
Deep: 150 – 200 m

Data Access

Signature Data File

Derived from these Archives

Seward Line near-surface nutrients plot
Nitrate plus nitrite (NO3+NO2) inventory within the surface 10 meters averaged
over different locations during spring (top) and fall (bottom).
Seward Line nutrient relations plot
The ratio of N:P of the average nutrient inventory from the surface 50 meters.

Records in this Signature Dataset illustrate the seasonality in nutrient conditions along the Seward Line from 2011 to 2020. The original measurements were made from water collected at stations from GAK1 to GAK13 during seasonal cruises each year. Dissolved inorganic nutrients include nitrate plus nitrite (N), phosphate (P), and silicic acid (Si). The availability of these nutrients influences primary production and phytoplankton community composition.

To calculate nutrient inventories, data have been integrated over a given depth at select stations, and then averaged by cross-shelf station location (station clusters). Nitrogen is a limiting nutrient in the Gulf of Alaska, and the N inventory in the upper 10 m provides a look into the seasonal and cross-shelf variability of this resource within surface waters. The N:P ratio shows the deviation from the Redfield ratio (N:P = 16) in the upper 50 m; an area of the water column that can potentially be mixed by storms during the growing season.

Data Column Contents:    

  1. Year
  2. Month
  3. Station_Cluster
  4. Depth_Layer: Depth layer over which nutrients are integrated
  5. N_[mmol/m^2]: NO3 + NO2 inventory of a given depth range (0 – 10 m or 0 – 50 m)
  6. Si_[mmol/m^2]: Silicic acid inventory of a given depth range (0 – 10 m or 0 – 50 m)
  7. P_[mmol/m^2]: Phosphate inventory of a given depth range (0 – 10 m or 0 – 50 m)
  8. N:P: The ratio of N to P in a given depth range (0 – 10 m or 0 – 50 m)
  9. Si:N: The ratio of Si to N in a given depth range (0 – 10 m or 0 – 50 m)

Station Clusters descriptions:
  Coastal: GAK1 and GAK2
  Mid-shelf: GAK5 and GAK6
  Shelf break: GAK8 and GAK9
  Slope: GAK12 and GAK13

Data Access

Signature Data File

Derived from these Archives

This dataset consists of average/integrated chlorophyll-a along the Seward Line from October 1997 to May 2021. The measurements were made at stations from GAK1 to GAK15 on water filtered from Niskin bottles spaced across the upper 50m of the water column. In later years (and variably before 2011), the phytoplankton was fractioned into cells greater than and less than 20 micrometers.  We report the fraction of large cells when such fractionation occurred.

Data Access

Signature Data File

Derived from these Archives