Nahant: Experimental Design

The first stage of the experiment attempted to determine what substrate factors were correlated with limpet abundance.  Substrate factors included: percent C. circumscriptum, percent Hildenbrandia rubra, percent canopy algae (Fucus, Ascophyllum, and Chondrus), and percent tide pool.  Quadrats (0.5x0.5m) were selected within the mid-low tidal zone (~25 m from splash zone) on relatively horizontal areas with significant amounts of bare rock.  Such areas were selected due to the relative rarity of limpets within the rocky intertidal and the need to choose areas likely to have multiple limpets (Figure 1).  As mentioned above, the percent of different substrate traits was visually scored for each quadrat.  A 4x4 grid superimposed on the quadrat was used to determine percent: 2 cells out of 16 would be 12.5%.

The second stage of the experiment focused on the distance between C. circumscriptum substrate and tortoiseshell limpets.  Plots were selected by locating patches of C. circumscriptum and placing the quadrat over them so at least one cell contained 50% or more algae.  The number of cells containing at least 50% C. circumscriptum was counted, and the number of cells containing any C. circumscriptum was counted.  Limpets were counted and scored based on the number of cells between them and cells with algae.  A limpet within a cell that contains C. circumscriptum receives a score of 0, while a limpet in a cell next to a cell that contains C. circumscriptum receives a score of 1 (Figure 2).