SeagrassNet monitoring summary

SeagrassNet = Seagrass monitoring Network

Goal: To measure change in seagrass:

Distribution via Position of seagrass relative to permanent transect (from the shore outward, with three 50m cross transects)

Species composition viaCollection along permanent transect

Abundance viaMeasurements of cover, canopy height, density, & biomass

Parameters to monitor and monitoring methods

Photographic record (one photo of each 0.25 m2 quadrat)

Email photos to

Voucher specimen

Prepare a herbarium sheet of each seagrass species (with flowering parts, if present) for the International Seagrass Herbarium. Send to SeagrassNet at the University of New Hampshire for verification, which will forward the sheets to the Smithsonian, Washington, DC, USA.

Seagrass cover (visual estimate of 0.25 m2 quadrat )

Estimate seagrass cover (total cover and then cover by each species) on a percent cover scale (0-100%) in each of the predetermined quadrats along the cross transects. Use the "Seagrass percentage cover photo guide.”

Canopy height

Measure canopy height ignoring the tallest 20% of leaves and identify any grazing evidence. Express results as canopy height for the dominant species.

Biomass: For small and medium-sized seagrasses use the 0.0035 m2 core

Take a biomass core to 10cm depth outside each quadrat by selecting an area approximately 0.5m landward of the quadrat in an area of the same seagrass species and cover as the quadrat (to your best judgment). Place the core over the selected shoots, making sure that leaves rooted in the core are on the inside of the core and those rooted outside are outside of the core. Wash the sediments from the core sample, separate into leaves, stems, and root-plus-rhizome. When processing, scrape epiphytes from the leaves and rinse the leaves in a weak acid if calcareous epiphytes are present. Rinse plant parts in fresh water, dry to constant wt (60° C for 24hr) and weigh. Calculate biomass (g dry wt m-2), enter on data forms, and submit data to SeagrassNet. Send the original of all data sheets to UNH; save a copy of the data sheets for your files.

For large seagrass species

Collect an individual shoot including rhizome approximately 0.5m landward of each quadrat. Determine shoot weight and multiply by density to obtain biomass.

Shoot density by species

From the biomass core, count the number of leaf meristems (points of leaf initiation on the shoot) occurring within the core sample and enter this number as the shoot count on the Biomass Form. For large seagrass species (e.g., large Thalassia plants) a 25 cm x 25 cm quadrat is placed on the transect line, and all shoots of the large species in the quadrat area are counted. Enter the number of shoots on the Field Sampling Form under the last Species (% Cover/Density) row.

Sexual reproduction

Count the number of flowers, fruits and seeds or flowering stems in a core for all seagrass species. Express results as flower/fruit/seed per area.

GPS: latitude and longitude

Take new GPS readings at the left, center, and right of each cross transect at every sampling event.

Distance to the seagrass edge and distance to the last shoot

The deep edge of the seagrass bed is defined as the deepest edge of the continuous meadow and the last shoot is the deepest occurring strap-bladed seagrass shoot.

      • Measure distance from the two ends and the center of the offshore cross transect (C) to the deepest edge and the last shoot.
      • The shallow edge of the seagrass bed is defined as the shallowest (most onshore) edge of the continuous meadow and the last shoot is the shallowest occurring seagrass shoot.

    • Measure distance from the two ends and the center of the onshore cross transect (A) to the shallowest edge and the last shoot.


Water depth is measured at 9 places (when the entire site is submerged) and the time is recorded.

• Using a depth measure (e.g., depth sounder, weight on a rope marked with depth, etc.) measure the depth from the top of the substrate to the water surface at each of the nine cross-transect sites (left, center, and right on the three cross transects).

Tidal information

Record what type of tides (diurnal, semi-diurnal) are found in the area and the tidal range (spring high to spring low) for the site (obtained from tide charts).

Record whether the minimum depth of distribution is in the intertidal (between the spring high and spring low tidal levels) or subtidal zone (below the spring low tide level).

Environmental data Water temperature (continuously measure temperature, °C) --iButton®

temperature data logger Salinity (measure at every visit and determine the range, ppt) --refractometer Light level (% surface light at deep and shallow depths) --Hobo® LI light logger Surface sediment characteristics quarterly at the center of the three cross-

transects --syringe corer

    • grain size (% gravel/sand/silt+clay) •organic content (% loss on ignition)
    • carbonate content (% loss after acidification)

Necessary equipment and materials (for one SeagrassNet site)

    • SeagrassNet monitoring manual

    • regional supplement to the manual
    • percent cover photo guide and species ID sheet
    • herbarium supplies and press
    • underwater digital camera
    • GPS unit (Garmon ETrek)
    • compass
    • quadrat identification photo labeler
    • 0.25 m2 quadrat (50 x 50 cm,) (2)

    • 0.0625 m2 quadrat (25 x 25 cm) (2)

    • waterproof data sheets (3 per site)
    • plastic clip board
    • 30 cm rulers (2), pencils, pencil sharpener, permanent markers
    • 50 m measuring tapes (3)
    • permanent station markers (9 screw anchors)
    • syringe sediment core samplers 20cc w/ rubber stopper (2)
    • salinity refractometer
    • plastic containers for salinity water samples (2)
    • HOBO Pendant light and temperature loggers (7)
    • HOBO launch and download software and cables
    • biomass core sampling tube 7cm diameter w/ 2 end caps
    • mesh bags for biomass (12)
    • large plastic ziplock bags (36 for biomass)
    • small plastic ziplock bags (3 for sediment)
    • paper bags (for drying biomass samples)
    • zip ties (20) and clipper for zip ties
    • rubber bands (25)
    • large plastic storage box for SeagrassNet kit
    • Also needed: use of a drying oven and a computer with Internet access

Sampling intervals

4 times/yr (January, April, July and October)

Establishing permanent sampling transects

Locations ranging from pristine (1 required) to stressed (optional)

One transect per location

Perpendicular to shore at the center of a 50m band running to the deep edge

3 cross-transects per transect, each 50m long with center points at:

    • Deep station (1 m into the bed from the offshore edge of the continuous meadow) mark with a permanent marker and record GPS location
    • Shallow station (1 m into the bed from the onshore edge of the continuous meadow) mark with a permanent marker and record GPS location
    • Mid depth station (between deep and shallow or at an interesting transition between species) mark with a permanent marker and record GPS location (with the assumption that cross-transects are independent)

Quarterly sampling

    • Go to your sampling site and find the permanent markers along the transect.

    • Retrieve the light and temperature sensors, take a salinity sample, and note the tidal stage on the data sheet (section 4.3).
    • At one of the central permanent markers, set up the cross transect using the 50m measuring tape to prepare for the sampling procedure along that cross transect (section 4.4). Avoid walking on the shoreward side of the measuring tape, as this is the area to be sampled.
    • Record information in the header of the datasheet and include any interesting observations (e.g., depth limit of species, etc.)

      • Sampling procedure for SeagrassNet quadrat measures (section 4.5)

          • Place a sampling quadrat (using the random distances provided on the datasheet) on the cross transect on the shoreward side. Position the quadrat so that the lower right corner of the quadrat
          • is on the random distance mark when the person placing the quadrat is facing the shore.

        • Take a photograph of the quadrat.
        • Identify the seagrass species present in the quadrat and enter the species codes on the data sheet.
        • Estimate % cover and % cover of each species in the quadrat using the "Seagrass percentage cover photo guide" in the back of the Manual.
        • Measure canopy height ignoring the tallest 20% of leaves. Enter on the data sheet and also enter evidence of grazing, if present.
        • Count the flowers and fruits, if present, and enter on the data sheet.
        • Collect a biomass core (or, for large seagrass species, individual shoot samples) outside the quadrat more than 0.5m shoreward of the quadrat in an area of the same seagrass species and cover as the quadrat (to your best judgement).
        • If large species, obtain a density count (0.25 x 0.25m quadrat) in the lower right-hand corner of the large quadrat.
        • Repeat (5. Sampling procedure) for each quadrat on the cross transect.

6. SeagrassNet cross-transect measures (section 4.6)

    • Collect voucher specimens (if not collected in the course of quadrat sampling) to create permanent herbarium samples, a minimum of one per species, with fruits and flowers if possible.
    • Measure the water depth at the 0m, 25m, and 50m points on the cross transect and, from these points, measure the distance to the edge of the seagrass bed. Record the water depths, the time, and the distances on the field data sheet.
    • Collect a sediment core sample at the permanent marker at the middle of the cross transect (20 cm from the marker). Note on the field data sheet.

7. Repeat steps 3 -6 for each station until all three cross transects have been sampled.

Post-sampling procedures Number of samples per station is 12 Number of samples per site is 36
    • Count the density of leaf meristems for each core (except for large species)
    • Measure leaf, stem, and root-plus-rhizome biomass for each core
    • Press the voucher specimens
    • Dry the sediment samples
    • Submit all data to SeagrassNet via the website:
      • Email the photographs, temperature and light data to or mail on CD.

      • Mail original datasheets (and keep a photocopy), dry sediment samples, and herbarium sheets to the Regional Node Coordinator (see SeagrassNet regional supplement) or to:
      • SeagrassNet Attn: Dr. Fred Short Jackson Estuarine Laboratory 85 Adams Point Road Durham, NH 03824 USA

      • Wrap up

        • Wash all gear in fresh water and dry
        • Review supplies for next quarter's sampling and request new supplies
        • Store gear for next quarterly monitoring


Download the SeagrassNet Monitoring Summary PDF here

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