4 Stratification Index (AZMP)

Data Type: Tabular Data

Spatial Scope: Scotian Shelf (4X, 4V, 4W)

Duration 1948-2024

Source: DFO Atlantic Zone Monitoring Program via azmpdata

4.1 Introduction to Indicator

Stratification Index describes the vertical density structure of the water column, typically driven by differences in temperature and salinity between surface and deeper waters. AZMP calculates stratification index as the difference in density between water at 0 and 50m depth, where density is derived from CTD profiles of temperature and salinity (Harrison et al. 2004).

A stratification index of 0 indicates no difference in density between surface and deeper waters, and implies the water column is well mixed. Higher stratification values indicate larger differences between surface and deep water, suggesting that surface water is warmer and/or fresher than deeper water.

4.2 View Data

plotly_df <- data@data %>%
  mutate(smooth_10yr = zoo::rollapply(mean_value, mean, width = 10, partial = TRUE),
         overall_mean = mean(mean_value))

p <- plot_ly(plotly_df, x = ~year) %>%
  add_lines(y = ~mean_value,
            name = "Yearly Anomaly",
            line = list(color = "lightgrey"),
            hovertemplate = "Monthly anomaly: %{y:.2f}<extra></extra>"
           ) %>%
  add_lines(y = ~smooth_10yr,
            name = "10-yr Smooth",
            line = list(color = "red", width = 2),
            hovertemplate = "10-yr smoothed: %{y:.2f}<extra></extra>",
           ) %>%
    add_lines(y = ~overall_mean,
            name = "Overall Mean",
            line = list(color = "black", width = 2, dash = "dash"),
            hovertemplate = "Overall Mean: %{y:.2f}<extra></extra>",
           ) %>%
  layout(
    title = "Stratification Index for Scotian Shelf (4X, 4V, 4W)<br><sup>Source: DFO Atlantic Zone Monitoring Program via azmpdata</sup>",
    xaxis = list(title = "Year"),
    yaxis = list(title = "Stratification Index (kg/m<sup>3</sup>)",
                 fixedrange = TRUE),
    hovermode = "x unified",
    margin = list( t = 80)
    
  ) %>%
  config(displayModeBar = FALSE) 

p

Figure 4.1: AZMP Stratification for Scotian Shelf; 1948-2024

4.3 Summary and Trends

Trend and summary values are automatically generated; data were last updated on marea package install on 2026-02-10

As of the most recent data entry in 2024, the Stratification Index is 0.03712, which is high among values in the timeseries. The Stratification Index is currently above the long-term mean, and has been since 2019. Stratification in the Scotian Shelf region has shown an increasing trend in recent years, and broadly throughout the time series.

4.3.1 Summary Table

Summary values for the AZMP Stratification in the Scotian Shelf region are shown below (Table 4.1)

Table 4.1: Summary values of AZMP Stratification in the Scotian Shelf region through time.

Metric	Value	Description
Most Recent Value (2024)	0.03712	The most recent value is high within the timeseries, in the 96.1 percentile of all values. The most recent value is 0.01 above the long-term mean.
Recent Trend (2014 – 2024)	3.9^{-4} per year	In the last ten years, azmp_stratification has increased by 3.9^{-4} per year with a Nonsignificant trend (p > 0.1).
Overall Trend (1948 – 2024)	1.6^{-4} per year	Since the beginning of the timeseries, azmp_stratification has increased by 1.6^{-4} per year with a Very Strong trend (p < 0.01).
Timeseries Record High	0.03982	The highest value in the timeseries was recorded on 2003, and was 0.01 higher than the overall timeseries mean.
Timeseries Record Low	0.00587	The lowest value in the timeseries was recorded on 1950, and was 0.02 lower than the overall timeseries mean.

4.4 Relevance to Research and Stock Assessments

Stratification can affect fisheries and marine ecosystems through bottom-up influence on primary productivity. Stratification can influence phytoplankton in the water column in complex ways. Stratified conditions can benefit phytoplankton by retaining them in the light-rich upper layer the water column. Alternatively, stratified water columns experience weaker vertical mixing, potentially reducing nutrient influx from deeper waters which can limit algal growth (Araújo and Bundy 2012). Expected effects of stratification change can therefore depend on the context of the target environment.

In the Scotian Shelf region, stratification index has shown a negative relationship with primary productivity (Araújo and Bundy 2012), and influence on the timing of season algae blooms (Song et al. 2011). Stratification can also affect water retention, thus potentially planktonic dispersal and larval transport.

4.5 Variable Definitions

Table 4.2: Column names and definitions in the AZMP_stratification_temperature dataset.

variable	description	unit
year	Year of data collection
region	Region over which samples are summarized (only scotian_shelf_box)
mean_value	Difference in water density between 0 and 50m depth	kg/m3

4.6 Additional Data

No additional data for AZMP Stratification.

4.7 Get the Data

library(marea)
data('azmp_stratification')
plot(azmp_stratification)

References

Araújo, Júlio Neves, and Alida Bundy. 2012. “Effects of Environmental Change, Fisheries and Trophodynamics on the Ecosystem of the Western Scotian Shelf, Canada.” Marine Ecology Progress Series 464: 51–67.

Harrison, Glen, Eugene Colbourne, Denis Gilbert, and Brian Petrie. 2004. “Oceanographic Observations and Data Products Derived from Large-Scale Fisheries Resource Assessment and Environmental Surveys in the Atlantic Zone.” AZMP Bulletin PMZA 4: 17–23.

Song, Hongjun, Rubao Ji, Charles Stock, Kelly Kearney, and Zongling Wang. 2011. “Interannual Variability in Phytoplankton Blooms and Plankton Productivity over the Nova Scotian Shelf and in the Gulf of Maine.” Marine Ecology Progress Series 426: 105–18.