Nahant_Discussion

Both the regression analysis and the graphical comparisons of the barnacle percent cover and average size compared with the six different variables can be used to assess which factors play a significant role in the distribution of S. balanoides in the intertidal zone. Starting with the regression analysis of the barnacle percent cover, only two variables were significant based off of their calculated P values. Wave exposure had a P value of 0.0375 and the surface of the substrate had a value of 0.0457. Since both had a P value below 0.05, they can be considered significant. Upon further graphical comparison, it was determined that there was a positive correlation between percent cover and wave exposure. The samples that were collected in areas that were completely open to the ocean and had very high wave exposure had the highest barnacle percent cover. On the other hand, the samples that were recorded in sheltered areas and had minimal wave exposure had the lowest percent cover. One explanation is that in areas that are completely exposed to the open ocean, the movement of water is very rapid and the amount of zooplankton and detris suspended in the water column is significantly higher than sheltered areas. Barnacles will use less energy feeding in areas where there is constant water movement. They feed by capturing zooplankton and detris in their thoracic like cirri which extend outward from the operculum. When the water is stagnant, the barnacles must rapidly beat their cirri in a rhythmic manner to propel the water around them and feed. This requires must more energy and the amount of food the barnacle can consume is limited because it can only propel a very small amount of water around it. In areas of fast water movement, there is a constant source of food for the barnacle and each wave brings water full of zooplankton and detris.

Alex and I returning to the bus at Nahant

The other variable that had a significant P value when compared with the percent cover was the surface of the substrate. The two variables were also analyzed graphically and the results showed a significant correlation between rough substrate and high percent cover. The average percent cover was nearly 6% higher on rough substrates as opposed to smooth substrates or smooth substrates with cracks. Since the average rough substrate had a percent cover of 19%, it was more a third higher than either of the other two substrates. From these results, it was determined that the barnacle percent cover increased on a rough substrate. One explanation for this is that barnacles permanently attached to their substrate and remain there throughout their life. It is likely that rough substrates are easier to attach to than smooth ones. Also, by permanently attaching to a rough substrate, the barnacle will be more tightly bound and is more difficult for predators to remove. Therefore, it is highly likely that attachment of the barnacle to a rough substrate confers several advantages.

It is also important to note the results of the comparisons between barnacle percent cover and slope, direction, and intertidal range. The regression analysis of these three variables yielded a P value greater than 0.05, which indicates that there is no correlation. However, upon further graphical analysis, it appears that there may be some significance. Because the samples were taken at random, some of the data obtained was somewhat skewed. For example, when the directions of the samples were recorded, there were many data points for East and South facing barnacles. However, there was only one data point for Southeast and four data points of West facing barnacles. Some directions had as many as 10 data points. This uneven accumulation of data points also occurred for the slope and intertidal range. From the resulting graphs, it appears that East and West facing barnacle masses had the highest percent cover. This correlation may be due to prolonged sunlight exposure. Also, it appears that the angle between 50-70º may be the preferred habitat of the S. balanoides, but more data is needed to draw any significant conclusions. From the graph obtained of percent cover compared to intertidal range, it appears that barnacles in the mid and high intertidal zone have a much greater percent cover than ones in the lower intertidal. This may be because there is much more competition in the lower intertidal. On the other hand, the barnacles would be covered by water longer in the lower zone and could feed longer each tide. Clearly, more data points for slope, direction, and intertidal range need to be recorded and analyzed to make any significant conclusions on their effect of barnacle percent cover in a given area.

The regression analysis of the average barnacle size against the six different variables also yielded significant findings. First of all, wave exposure and slope yielded significant P values, 0.000625 and 0.0309, respectively. The value obtained for wave exposure was very significant because it was well below 0.05 and R squared value was 0.24. The correlation between wave exposure and average size was the strongest correlation obtained in this study. The exposure was also compared with the average size graphically. The results showed a clear positive correlation. As the wave exposure increases, the size of the barnacle also increases. In the sheltered areas, the average size of the barnacles was the smallest. One reason for this correlation is that in areas that are openly exposed to the ocean and rough waves, the barnacles must have a strong, permanent attachment to the substrate to prevent being ripped off and washed away. Larger barnacles would be more adapted to living in this harsh environment. Another reason why the barnacles are larger here is because of increased wave action. As mentioned before, the faster the wave action, the more food that is available for the barnacle and less energy is required to feed.

The regression analysis of the average size and the slope also yielded a significant P value. The results were further analyzed graphically, but it is difficult to make any certain conclusions. The slopes appear to be spread out relatively even and it’s difficult to determine whether there’s a positive or negative correlation. Also, it appears that no one slope is preferred over another. More slope data points need to be obtained before any conclusions can be made.

Intertidal range, direction, and the surface of the substrate were also graphically compared with the average size. The substrate surface had no affect on the average size of barnacles. A graph of intertidal range compared with average size showed that larger barnacles were found in the mid intertidal zone over the low and high zones. One explanation for this is that the mid intertidal zone in covered by water a considerable amount of time each tide, so barnacles located here can feed longer than the higher ones. Also, they face less competition than the lower ones and therefore grow to be larger. Finally, the direction of the barnacles was compared with the average size and the graphical results show that there is no correlation between the two. Because the data was skewed, a correlation could potentially exist, but more data is needed.

The temperatures of the samples were recorded and analyzed, but they did not yield significant results. The reason for this is that the samples were collected on three different days and the weather changed each time. The first day the samples were collected was in September. The second and third trips were in November. The first trip was a sunny day, the second was right at sunset, and the third trip was a warm rainy day. The temperatures fluctuated from 4-17ºC and no conclusions could be made based on the temperature of the barnacles and their substrate.

In the study, both regression analysis and graphical analysis were utilized. From the results, it was determined that a high percent cover of barnacles was strongly influenced strong wave exposure and a rough substrate surface. Furthermore, it was also concluded that the average barnacle size in an area is strongly influenced by wave exposure. The results indicated that the slope of the substrate had a correlation with the average size, but the correlation was quite determined. In conclusion, this study found that high wave exposure was most influential variable that increased both average size and percent cover of barnacles.