Variation in ecosystem structure as a result of swell exposure
Liam J. Zarri
Abstract
The exposure of a marine ecosystem to oceanographic forces may be a factor influencing community structure. To test this, two marine reserves with different levels of exposure to ocean swell were surveyed on SCUBA. Both sites receive similar amounts of upwelling, and both are rocky reef kelp forests with strict no fishing regulations. Fish, algal and invertebrate assemblages were sampled on a day with low swell then a day with high swell. Results showed that algae and invertebrate species were significantly different at the two sites, with some species favoring the site with high swell exposure and others favoring the more protected cove. Fish displayed an interaction effect by showing site specific behavior between the two days.This implies that swell conditions, affecting the exposed site more, have an effect on fish performance. These findings support the hypothesis that the exposure of a marine ecosystem to ocean swell is a factor influencing community structure.
Introduction
Coastal marine ecosystems respond to different kinds of disturbances. Exposure to oceanographic forces such as storms, current and swell may be one of the most important factors influencing community structure on both the geographic and local scale. For example, coastal upwellingis caused by consistent winds traveling parallel to the coastline combined with current. The California Current is an Eastern boundary current, and when combined with predominating Northwesterly winds common along the coastline, causes productive upwelling (Mann & Lazier 2006). This eventbrings nutrients into shallow coastal ecosystems, which are then harnessed by photosynthetic plankton. The energy then continues into higher trophic levels through the food chain (Ryther 1969).Other physical forces have more local effects, resulting in diversity between ecosystems which are subjected to similar degrees of upwelling. Differences in light, turbidity, water temperature, pH, depth and swell produce distinct sites which may be located close to one another. Sites which are exposed to swell and ocean storms can undergo dramatic ecosystem wide changes over the course of a few years (Ebeling et al, 1985). We sought to quantify community differences between two similar sites which varied only by exposure to swell.
The kelp forest is one of the most productive ecosystems on earth. It thrives in cold, nutrient rich waters such as those affected by upwelling. Physical structures provided by a kelp forest, such as canopy, midwater, and understory levels, support a large diversity of fish and invertebrate assemblages on which many terrestrial species depend. For example, coastal cormorants, pelicans, and many other coastal birds actively hunt within the subtidal kelp forest. Kelp forests also support many ecosystem services such as fisheries, primary production, pharmaceuticals, nutrient dispersal and recreation. However, subtidal forests are subject to sources of physical disturbance such as wave action. Increased water movement can rip up kelp individuals, stir up previously untapped nutrients on the bottom, and encourage larvae dispersal. Therefore, it is important to understand how the kelp forest community structure and function responds to oceanographic disturbance.
Hopkins Marine Station and Whalers Cove on Point Lobos are two kelp forests located in southern Monterey Bay, California. Although they are separated by a small distance, the kelp forest communities at each site are very different. Our study quantifies differences between these two sites. Hopkins is protected from swell, while Whalers Cove is exposed near the tip of Point Lobos (Fig 1). Therefore, these two sites are ideal to test the effect of swell exposure on community structure.
Figure 1. Map of Point Carmel and Point Lobos and the State Marine Reserves in the area. Hopkins is indicated by the red cross in between the Edward Ricketts SMCA and the Pacific Grove Marine Garden SMCA. Whalers Cove is the marked as the green cross in the Pt. Lobos SMR. Note the exposure of both sites to swells. Hopkins is in an isolated nook of the Monterey Peninsula and Whalers is almost at the tip of Pt. Lobos.
Anthropogenic forces play a large part in structuring marine ecosystems. Water quality, climate change, kelp harvesting and fishing all have dramatic impacts on kelp forest communities. The Monterey Bay contains many marine reserves, which do not allow fishing. Marine reserves are a type of marine protected areas (MPA). The introduction of an MPA into an area, and associated restriction of fishing, can have a dramatic positive impact on the health of kelp forest, when examined against an unprotected area (Ling et al, 2009). In light of the strong effects fishing may have on a kelp forest, two marine reserves were the platforms for our study.
To answer to overarching question of community differences between our sites, we propose three hypotheses.First, thatvariation exists in the abundance of species between two sites with different swell exposure. To test this, we analyzed differences in fish, algae and invertebrate abundance between both sites. Second, we suggest that the day sampled had an impact on species assemblages.Species abundance was recorded over two separate days, one with low ocean swell and the other with high. Our opportunistic study took advantage of the change in swell to examine how species abundance responded to increased water movement. Third, we propose that there an interaction effect of both site and day on species abundance. Taxonomic groups were assessed independently for discrete results.
Methods
General approach
In order to compare fish, invertebrate and algae assemblages between Hopkins Marine Station and Whalers Cove, quantitative transects were conducted on subtidal rocky reef communities on SCUBA.. Surveyors recorded 29 different species of algae, fish and invertebrates to evaluate differences between taxons. Data was analyzed using SYSTAT and PERMANOVA statistical tests for multivariate analysis. Multi Dimensional (MDS) plots were used to graphically represent data for each hypothesis.
Study system
Hopkins Marine Station through Stanford University is a Marine Reserve located in Pacific Grove, California (36®36’N, 121®54’W). Shallow areas (0-6 m) are composed of large granite benches with high cover of small turf algae and high turnover of giant kelp,Macrocystis pyrifera. Deeper zones (8-13 m) contain high-relief pinnacles encompassed by shell rubble in deeper water. The deeper sections have significantly less turf algae cover with lower turnover rate of Macrocystis and higher density of understory kelps (Watanabe 1984). Hopkins is nestled in northern nook of the Monterey Peninsula making it one of the most protected sites in the area (Fig 1). The cove is located facing towards inland Monterey, thus removing it from the path of all but the strongest northwestern swells.
In contrast, Whalers Cove is a Marine Reserve located on the tipof Point Lobos in south Carmel, California (56®51’875”N, 121®95’2475”W). The covehas a wide mouth, allowing west to northwest swells, which are common during the fall, winter and spring, to penetrate into the inlet.The inner cove is composed of a shallow (~8m) sandy seafloor which slopes gently down towards the more active cove mouth. Macrocystis grows on rubble along the edges of the site. Sand channels leading towards the mouth turn into boulder and cobble slopes which originate at middle reef, which splits the cove in two. Macrocystis, Pterygophora californicaand Southern sea palm, Eisenia arborea cover the productive outcrop.
Surveys were conducted at both Hopkins and Lobos on October 11th, 2011 and October 13th, 2011 in the morning, in between 9 and 10 AM. The NOAA oceanographic buoy number 46042 located 27 nm off Monterey recordedawest-northwest wave height of 1.8 meters at an 11 second periodat 0900 on October 11th (NOAA National Buoy Data Center). Both sites reflected this small swell. A strong northwest swell hit Monterey on October 12th but began to quickly taper off. October 13th showed the remnants of the swell at 3.0 meters at 0800. However, the long period of 19 seconds led to a powerful swell affecting both sites. Whalers Cove was more exposed to this swell, resulting in large waves inside of the cove. Due to its more protected location, Hopkins experienced a less active swell, with smaller waves resulting in lower turbulence within the kelp forest.
In order to quantify differences between Hopkins and Whalers Cove we sampled algae, fish and invertebrate assemblages (Table 1). Species were chosen to be representative of the taxa so that the abundance of a few key species may indicate abundance of a greater part of the community. In order to qualify, individuals also had to be countable, as species were measured based on quantitative abundance. Therefore a species also had to present in high enough abundance as to achieve statistical power. Finally, we chose species that were recognizable, and therefor countable, asto avoid bias based upon mistaken identification.
Table 1. Species surveyed at Hopkins and Whalers Cove broken up by taxa.
Algae / Fish / InvertebratesCystoceira osmundacea / Oxylebius pictus / Patiria miniata
Chondracanthus corymbifera / Hexagrammos decagrammus / Pycnopodia helianthoides
Dictyoneurum californica / Sebastes mystinus / Pisaster brevispinus
Macrocystis pyrifera / Sebastes carnatus / Pisaster giganteus
Dictyoneuropsis reticulate / Sebastes chrysomelas / Urticina piscivora
Pterygophora californica / Sebastes atrovirens / Urticina lofotensis
Eisenia arborea / Embiotica jacksoni / Pachycerianthus fimbriatus
Embiotica lateralis / Balanophyllia elegans
Damalichthys vacca / Tethya aurantia
Calliostoma ligatum
Loxorhynchus crispatus
Haliotis rufescens
Strongylocentrotus fransiscanus
Sampling design
In order to reduce variability between sampling methods, buddies recorded data using the methods at Hopkins as at Lobos. Depth was also relatively consistent between the two sites, with a median depth at ~11m (Figure 2). Surveyors at Hopkins were arrayed in 10 meter increments along the permanent transect line. Divers ran a deep transect at a 90® heading, collecting fish data as we ran out the meter tape. We sampled for invertebrates on the swim back to the main line, each buddy surveying their half of the transect. Algae abundance was recorded while returning to the reel, after which the tape was then rolled up. The same process was repeated for the shallow side of the main line.At Whalers Cove, a temporary main line was deployed running parallel on the eastern side Middle Reef, which runs approximately north-south. Surveyors ran transects at 10 meter increments towards Middle Reef, collecting fish, algae and invertebrate data. Instead of sampling on the opposite side of the main line, surveyors moved down 5 meters and repeated the same process.
Figure 2. Depth of transects at Hopkins and Lobos. Note the wider first lower quartile at Hopkins is due to the shallow transect performed on the inland side of the main line.
Data analysis
In order to answer our question of variation in species assemblages between site and day, statistical tests were performed to analyze our data. PERMANOVA analyzed variance between site and day while Multi Dimensional Scaling (MDS) plots graphically depict the results.
Is there a difference in species composition between Hopkins and Lobos?
In order to test abundance, algae, fish and invertebrate assemblages were quantitatively sampled at both sites. Each buddy performed a 1 by 30 meter transect on their respective side of the tape to sample for invertebrates and algae. To record fish data, surveyors counted all fish which passed through a 2 by 2 by 2 meter cube of waterfor the length of the 30 m transect. Replicates for this hypothesis were transects within each site.
Is there a difference in species composition as a result of day?
In order to test abundance, algae, fish and invertebrate assemblages were quantitatively sampled on both October 11th and October 13th. Replicates for this hypothesis were transects for each day.
Is there an interaction effect on species composition as a result of both site and day?
In order to test for an interaction effect, algae, fish and invertebrate assemblages were quantitatively sampled on both October 11th and October 13th at both Hopkins and Lobos. Replicates for this hypothesis were transects on each day and at each site.
Results (This is not right on multiple levels. 1) The results section is NOT a laundry list (write out the results and include the logic for the tests to make the results cohesive) 2) if you consistently reference a figure- like figure 3. Then writing out the statistic in the text is redundant and a waste of space.
State your result and reference the table or figure or analysis. E.g. Species composition differed between Pt. Lobos and Hopkins (table 1).
If you need clarification, find one of us to help you.
Significance was determined by a score of <0.05, or 5% on the PERMANOVA test.
Is there a difference in species composition between Hopkins and Lobos?
We found a strong site effect on the species composition of the algal assemblage (Fig 3; PERMANOVA: site effect, P=0.0001).
We did not find a strong site effect on the species composition of the fish assemblage (Fig 3; PERMANOVA: site effect, P=0.319)
We found a strong site effect on the species composition of the invertebrate assemblage (Fig 3; PERMANOVA: site effect, P=0.0001)
Is there a difference in species composition as a result of day?
We did not find a strong day effect on the species composition of the algal assemblage (Fig 3; PERMANOVA: day effect, P=0.724)
We found a strong day effect on the species composition of the fish assemblage (Fig 3; PERMANOVA: day effect, P=0.043)
We did not find a strong day effect on the species composition of the invertebrate assemblage (Fig. 3; PERMANOVA: day effect, P=0.505)
Is there an interaction effect on species composition as a result of site and day?
We did not find a strong interaction effect on the species composition of the algal assemblage (PERMANOVA: interaction effect, P=0.926)
We found a strong interaction effect on the species composition of the fish assemblage (PERMANOVA: interaction effect, P=0.015)
We did not find a strong interaction effect on the species composition of the invertebrate assemblage (PERMANOVA: interaction effect, P=0.569)
Figure 3. Percent of variance displayed betweenexplained by day and site for each taxon. Percentage values come from PERMANOVA statistical test.
Adequacy of sampling design
The power of a statistical test is the probability that the results will reject the null hypothesis if it is actually false. To assess whether or not our project had statistical power, three factors must be taken into account. First, what are our optimal numbers of sampling days, and does this vary by taxon. Second, how many transects do we need at a particular site to achieve statistical power for each taxon. Finally, are there differences between species which afford them greater or lower statistical power.
Algae and invertebrate taxon showed no day effect. To test our hypothesis, we needed only to capture a snapshot in time of both communities, so we can state with confidence that we sampled an adequate number of days for both taxon. Fish assemblages showed a strong day effect, which could have been the result of a true day effect or bias resulting from inadequate number of sampling days No: there was an interaction between site and day…meaning that a day effect is not discernable from those stats. Also, figure 4 indicates that fish were undersampled….no asymptote.. However, with a power exceeding 2 (Fig 4) we can state with reasonable confidence that we sampled for an adequate number of days.
Algal assemblages were sampled for an adequate number of transects (Fig 4); in fact, we could have sampled less transects (~24) to achieve a higher power. We did not sample enough transects for fish assemblages, and statistics only gives us a ballpark figure as to how many more transects were necessary to achieve asymptotic power. The invertebrate power index shows an asymptote at 15-20, which represents the number of transects needed at Hopkins. However, the line continues upwards, indicating that not enough transects were sampled to achieve statistical power for invertebrates at Point Lobos.
Figure 4. Power index by number of transects of each taxa. Green represents algae (asymptote=24), blue represents fish (no asymptote) and red represents invertebrates (first asymptote=15-20 represents Hopkins, but second asymptote for Lobos not achieved)
Different species have varying levels of statistical power. Abundant species will be counted more often (such as giant kelp, Macrocystis pyrifera) and therefore have higher power. The opposite holds true for rarer species.Recognizable species are more likely to be recorded every time they are encountered, resulting in higher power. The black and yellow rockfish, Sebastes chrysomelas, was relatively rare but its recognizable coloration gives it high power. Species that live or forage out in the open are easier to notice, such as the bat star, Patiria miniata. A crack dwelling species such as the red urchin, Strongylocentrotus fransiscanus, will have a much lower power.
You have nothing in here about the species that contributed to the differences between sites….a very main point
Discussion
Results showed a strong association of algae to each site. Lobos displayed an abundance of tough, woody kelps such as Pterygophora californica and southern sea palm, Eisenia arborea, while both species were absent at Hopkins. Fleshier plants with higher surface area (and therefore a higher drag ratio) such as Turkish towel, Chondracanthus corymbifera, and Dictyoneuropsis californica were abundant at Hopkins and absent at Lobos. These observations achieve significance when we look at site exposure. Hopkins, in its protected nook of Monterey, favors recruitment of fast-growing plants with high surface area, which can outcompete slower growing, tougher species. When these types of plants recruit to Lobos, rough conditions prevent growth and these species possibly get destroyed in large swells. This allows recruitment and growth of tough understory species.