Characterizing Environmental Gradients at Garrison Bay

the three transects used during sampling

An environmental gradient is a zone of transition from one set of biological and physical conditions to another that is defined by a parameter(s) (e.g. depth, oxygen content) that changes at a measurable rate over a known distance. This distance can range from a few millimeters to many miles. At smaller scales, environmental gradients are useful because they represent a compact succession of overlapping habitats, each varying relative to the others in a moderately predictable way. Because the change in one or more parameters per unit of distance is known, distance may be used as a proxy for time along the gradient. Thus, environmental gradients are ready-made laboratories for studying ways in which organisms adapt to changes in environmental conditions over time.

Mud Gradient - Primary Transect (A)

Laying out the primary transect (A - A'), due north and in line between the bench on the Howe property and the left-most building (part of British Camp)

Garrison Bay contains an environmental gradient across the entire intertidal zone. Our primary transect (A-A') covers most of this gradient, beginning at the shoreline and running well into the lower intertidal. The transect is characterized by a gentle muddy slope, low oxygen conditions at relatively shallow depths within the sediment, diatomaceous ooze and algal clumps near the shore, patches of polychaete castings, and abundant burrows and mollusc shells. An important sediment feature is a layer of dense, resistant sub-surface material--clay and/or pebbles--that varies in its depth below the sediment across the intertidal.  Based on our initial survey of the site, we considered a range of surface and subsurface variables prior to examining the primary transect in detail.

At sediment surface
- Relative thickness of diatomaceous layer
- Frequency of algal clumps
- Percentage of shell cover
- Frequency of burrows (small and large)
- Frequency of polychaete fecal material
- Frequency of tubes (Phoronid and polychaete)
Within sediment
- [Oxygen content]
- Temperature
- Depth to resistant layer
- Percentage of coarse material
- Faunal variation between shell
   infill and surrounding sediment

Oxygen content could not be measured within the sediment due to the sensitivity of the measuring device (mud would disrupt the sensor and damage the fragile membrane).  Based on our observations (sight and smell!) we concluded that the distance from the sediment surface to anoxic sediment layers was shallow and remained relatively constant over the length of the transect.

Clinocardium nutalli valves with associated polychaete, decapod, and gastropods
During our initial survey of Garrison Bay, we found polychaete worms (commonly capitellids, glycerids, and hesionids) on the inner surfaces of dead bivalve shells that had been filled with sediment. Often, these worms were clumped together in masses of five or more individuals, many of them juveniles. In some cases, brittle stars (Amphipholis squamata) and crabs (Pinnixa sp.) were also present.  The frequency of observed shell-dwellers led us to speculate that dead shells in Garrison Bay could serve as microhabitats and/or refuges for a variety of small benthic organisms.  If this is true, then the faunal composition of shell infill could differ from that of the surrounding sediment.


All data collection took place at Garrison Bay during a series of exceptionally low tides, May 31 through July 3, 2004.

We subdivided the 37.5-meter primary transect into 75 successive quadrats, each a 0.5-m wide. Quadrats were laid out to the right of a rope line, and each was photographed. The depth to the resistant layer was measured for each quadrat using a meter stick.

Surface Analysis

Each quadrant was visually scored as follows:
- % shell cover
- # small burrows (< 1cm diam.)
- # large burrows (> 1cm diam.)
- thickness of diatomaceous layer

Presence/absence noted for the following
- algal clumps
- polychaete fecal matter
- live organisms on the surface
- tubes (phoronid, mucus/sand, or mud)

Subsurface Analysis

We used two sampling techniques to assess the variability of sediment composition and faunal diversity/abundance throughout the transect.  Both techniques were used within every fourth quadrant along the transect, resulting in a total of 36 samples:

Team Mud & interhabitat  liaison collect data

Large and small burrows

Surficial sediment, showing green-brown diatomaceous layer over gray-black sediment


Tec hnique 1: Sediment Cores
Core samples were taken using a simple cylindrical device called a "clam gun." For each core, the clam gun was pushed down into the sediment until it came in contact with the resistant layer. (Sediment volume varied among the cores as a factor of the depth to the resistant layer.) Then, the "gun" was removed, along with a cylindrical sediment sample.

Core samples were sieved using three mesh sizes: coarse (5 mm), medium (2 mm), and fine (500 µm). The total volume of each core was calculated prior to sieving. After sieving, the volume of the coarse fraction (material caught by the 0.5 mm sieve) was calculated and both raw volume and the proportion of coarse material were recorded. Then, each sediment size class was examined for the presence of invertebrate material. We identified living and dead organisms, shells, and mucus tubes and recorded their abundances within each size fraction.

Technique 2: Shell/No-Shell Comparisons
Five dead shells of approximately equal size were collected from the sediment surface in every fourth quadrant (every 2 meters). The sediment within each shell was retained and left undisturbed. Then, one empty shell of comparable size was used to collect five shells' worth of sediment. Shells from each quadrat were placed in a bag and subsequently compared with the corresponding "no-shell" sediment.

To measure and record sediment temperatures over several days, we used small data loggers (iButtons). Five iButtons were mounted vertically along the lengths of each of two PVC tubes, which were buried perpendicular to the surface. One tube was buried in the sediment at the onshore end of the primary transect and the other at the offshore end. Burial took place on July 1 at 8:35 a.m. The tubes were recovered on July 3 at 12:06 p.m.

Mud Gradient - Additional Transects (B,C)

transects B and C were used for focal sampling

Visual inspection of the habitat showed that four faunal elements--"gaper" clams ( Tresus capax), anemones, and the egg masses of two gastropod species (Melanochlamys diomedia and Haminoea visicula --were relatively common (though patchy) in the lower intertidal, but either rare or absent along our primary transect.  For faunal elements like these and others distributed in a narrow band of the intertidal, our sampling along the primary transect proved inadequate. Despite adding a second dimension to our sampling area by using quadrats, we covered only a narrow swath of habitat relative to the length of the shoreline. In order to gain a better sense of faunal diversity in the lower intertidal, we established two additional transects (B-B' and C-C'), both oriented roughly parallel to the shoreline. Three people walked each transect side-by-side and two arms' lengths apart, counting all visible gaper clams, anemones, and egg masses along. We compared the totals from each quadrat to assess differences within the lower intertidal.

[Garrison Bay Main Page] - [Home]