Methods for Characterizing the Cobble Beach

The tidal gradient presents both interesting benefits for the animals found in this habitat and some important challenges for a visitor interested in knowing the animals. We made two types of observations-the first was based on tidal regimes and the second on diversity of animals under rocks of different sizes.


Observation 1: Tidal Regimes

The objective was to quantify changes in species number and abundance across the environmental gradient established by the tides. To accomplish this goal, we ran 6 transects (a line of rope with graded marks from the highest part of the beach to the water line at the time of the low tide) through the intertidal area, 3 on each beach, spaced evenly apart. Starting 7 meters from the high water line, we marked out 20 meters to the water. We sampled the animals along each line using standard-sized small square grids (or quadrats). We laid our last quadrat at the edge of the low tide mark. Our fieldwork coincided with a series of exceptionally low tides, thus we were lucky to get data for the lowest quadrat, which would normally be underwater at most other low tides during the year. We laid quadrats every 4 meters, which gave us 5 heights (which we call Zones 1-5) along the beach.

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The sheltered beach was steeper and shorter from the water to the top of the beach, thus only 4 quadrats were laid along 1 of these 3 transects.

In each quadrat we noted the relative number of rock sizes, and recorded the fauna above, below, and under the first layer of rocks. If a rock was partially in the quadrat, it was included in the sampling.


Observation 2: Diversity under different sized rocks

The objective was to quantify changes in species number and abundance between two rock sizes: small (20 cm²) and large (40 cm²). To accomplish this goal we ran two 16 meter transects through the exposed beach starting from the low water mark. We chose two independent small rocks and two independent large rocks at each height, for a total of 16 rocks per transect, 32 in all. For each rock we characterized the fauna above and below the rock only. We also noted the proportion of the rock surface covered by algae and by barnacles.

 

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