Subtidal Ecology of Frenchman Bay Establishing long-term monitoring sites J. Alex Brett College of the Atlantic Spring, 2011
Abstract Long-term monitoring programs are an essential component of understanding population dynamics, particularly in response to anthropogenic activity. The marine environment of Downeast Maine is subject to a wide range of factors resulting from human actions, including commercial fishing, climate change, and species introductions. In order to provide baseline data to understand the impacts of these actions I used photoquadrats to survey the rocky subtidal zone at two sites in Frenchman Bay, Maine. The data demonstrated interesting trends in population distribution patterns in the Bay; I found high levels of variation both between sites and between individual quadrats within sites. The site at Bar Island had a significantly higher density of filamentous red algae than Long Porcupine. As predicted by past studies, percent cover estimates of filter feeders were found to vary significantly between horizontal and vertical surfaces. The data from this study support earlier conclusions about marine invertebrate population patterns, and indicate the importance of fixed plots for long-term surveys to reduce the effect of environmental variation on survey results.
Introduction
At present there appears to be little current understanding of the population dynamics of the benthic marine life of Downeast Maine, which is a knowledge gap that needs to be addressed. The bulk of population studies on marine life have focused on commercially important species, such as scallops (Hart 2006) and bony fishes (Brodziak and Traver 2006,
Mayo and O’Brien 2006), or ecologically sensitive species such as eelgrass (Disney and
Kidder 2010). From a management standpoint this is inadequate, because NOAA has been advocating the implementation of ecosystem based fisheries management for more than ten years (NMFS 1999), which requires a near-complete understanding of the ecosystem (Link
2002). On an even more fundamental level, long-term ecological data is an essential tool for understanding changes in the natural world (Strayer et al. 1986, Mikkelsen and Cracraft
[2]
2001, Magurran et al. 2010), and it is a tool that is all too often ignored in favor of studies providing immediate results.
Due to the difficulty of observing organisms underwater, estimates of marine populations have primarily relied on indirect methods, such as trawls or landing data from commercial fisheries (Pearson and Barnett 1987, Chen et al. 2005, Brodziak and Traver
2006). When combined with models, these can give a ballpark figure for populations, but since there are never direct observations of the organisms in question, the models are not completely reliable (NRC 2000, Rose and Cowan 2003). The indirect survey methods used by fisheries managers are generally conducted with the same gear used in the fisheries
(Kilduff et al. 2009), which means that species of no commercial importance will generally not be accounted for in the surveys. These indirect methods are generally destructive to the organisms being studied, and given the current overstressed state of the ocean (Bax et al.
2003, Myers and Worm 2003, Feely et al. 2004), nondestructive survey methods are preferable.
When designing a long-term monitoring program, it is important to make sure that the area being studied is being accurately represented. If a particular portion of a habitat is not included, that may have dramatic impacts on the results of the study. In the rocky subtidal, not all areas of rock are created equal; horizontal and vertical surfaces can have a very different population of species (Sebens et al. 1999, Miller 2005). The reasons for this are varied, but two major factors are sedimentation and quantity of light that reaches the ocean bottom (Miller and Etter 2008). In order to properly understand a rocky subtidal site through a long-term monitoring program, the differences between horizontal and vertical rock faces should be studied and accounted for by a diversity of microhabitats.
[3]
The data that has been gathered in Frenchman Bay to date has given little focus to the rocky subtidal zone. There has been some work on historical fisheries (Alexander et al. 2009) and dredgeable surfaces in the subtidal (Procter 1933), but data around the rocky subtidal is scarce. A major factor in this may be that remote monitoring of such a habitat can be difficult, because of the expense of using a remote operated vehicle (ROV) and because of the inefficiency of more conventional sampling gear on rocky surfaces. To this end, in the following study I have attempted to create a baseline dataset on the invertebrate community of Frenchman Bay using direct observation by SCUBA divers; these data can be used to examine small-scale distribution patterns in algae and invertebrates. In addition the data will allow future researchers to study long-term changes in population.
Methods of Censusing Marine Populations
Early methods of sampling and quantifying marine organisms relied on a range of indirect methods, such as dredges and grab samples (Robson 1923, Procter 1933). These methods were generally effective, and are still in common usage (Pearson and Barnett 1987).
However, there are two major issues with indirect sampling that makes more direct methods preferable.
First, indirect sampling is destructive to the organisms being studied and to the environment they inhabit. There have been concerns about damage caused by trawls almost as early as they were first used (Jones 1992). In a number of areas, chronic trawling has led to large shifts in the structure of benthic communities (Watling and Norse 1998, Jennings et al. 2001, Thrush and Dayton 2002). It should be noted that these are the results of heavy
[4] commercial fishing, and research trawls do not approach that level of frequency, though they may still cause damage at low intensity.
Additionally, indirect methods can be unreliable in a number of ways when compared to direct methods of observation (Lessios 1996). Catch rates for some methods, such as dredges, can be quite low; on the order of 11% for scallops (Mcloughin et al. 2003), and 2-
32% for oysters (Chai et al 1992). This level of variation can make it difficult to compensate for the low catch rates and would likely lead to inaccurate population estimates. Another factor to be considered is that some habitats are more favorable to trawling, and when species are non-randomly distributed between habitats there can be a bias in the population estimates
(Jagielo et al. 2003). Trawls and other indirect methods are useful tools, but when possible direct, nondestructive observation should take preference.
Remote operated vehicles (ROVs) provide in situ observations of marine life down to the deepest regions of the world’s ocean (Bowen et al. 2009). Unlike trawls and other indirect survey methods they can provide direct evidence on the abundance of organisms as well as behavioral information. There is of course the potential for an observer effect on the organisms being studied (Spanier et al. 1994, Stoner e al. 2008), which could impact both the behavior and the numbers being reported. To date, the majority of ROV work has been conducted at depths beyond the range of compressed air SCUBA diving (eg. Krieger 1993,
Newman and Stakes 1994, Reed et al. 2005, Kahng and Kelley 2007). However, recently they have been used more frequently at depths safely reachable by SCUBA divers (Lirman et al. 2006, Lame et al. 2007). ROVs do not run the risk of decompression sickness or other pressure related injuries that divers must be wary of, which makes them an excellent choice
[5] for surveys from a safety perspective. However, there are several factors that currently make
ROVs unsuited for most shallow water operations.
One of the primary disadvantages of using ROV for surveys of marine life is their high cost. A simple model rated to 150 m from Seabotix Inc. starts at $29,995, and when extra features are added that price could easily rise by $50,000 or more (Stephanie Arbridge,
Advex Marine, 2011, pers. comm.). A similar ROV system from Videoray, another large manufacturer of ROVs for research and commercial use, starts at $43,495 (Paul Igo,
Oceanographic and Geophysical Instruments, 2011, Pers. Comm.). Equipment can be leased to avoid the high costs, but costs can still be in the neighborhood of $1,000 a day for a relatively basic model (Pacunski et al. 2008). An additional complication with ROVs is the high level of logistical support needed during operation (Pacunski et al. 2008). Some recent work has shown ways for small ROVs to be operated with a minimum of surface support
(Csepp 2005), but they still generally require more logistical support than operations using
SCUBA. As technologies develop and advance, the price of ROVs may decrease and handling operations may become easier, but at present those difficulties generally restrict
ROVs to surveys beyond the depth of air-based SCUBA divers.
Like ROVs, SCUBA diving allows scientists to directly observe target organisms without needing to kill or damage them first. Additionally, diving-based operations are relatively cheap compared with the expense of renting or purchasing an ROV. However,
SCUBA diving has its own set of restrictions that limits its usefulness for research. Dive- based surveys are incredibly time consuming in comparison with trawl or dredge-based studies. Diving also involves an increased danger to the participants, which means that safety parameters must be built in to prevent harm and that further reduces the amount of time that
[6] can be spent conducting a survey (AAUS 2006). Like many other methods, the presence of the diver can induce an observer effect in the organisms being studied, though that is mostly of concern for highly mobile organisms such as fishes (Dickens et al. 2011).
There is no one best technique for censusing marine populations using SCUBA. Each environment and study organism has its own set of quirks that require fitting research methods to individual project locations. Thus, any attempt to summarize all methods would either fail or become an unreadable tome, so I will settle for a brief comparison of some of the more common techniques. The three primary categories that most surveys fall into are video, photographic, and visual. Visual surveys generally require the least equipment beyond basic SCUBA gear, which makes them attractive for low-budget operations or situations where large numbers of volunteers are engaged in data collection (Koss et al. 2005). They also are generally more accurate than videos or photographs (Murray 2001, Tremblay et al.
2005). However, they can be time consuming and require all researchers to be highly familiar with the marine flora and fauna of the region (Murray 2001).
In comparison with visual surveys, photographic and video-based censuses tend to be more costly, because of the additional electronics required. However, they have advantages that can make the added expense worthwhile. Photographs provide a permanent record of the area being studied (Sebens et al. 1997), which allows additional analysis to be performed at a later date. Digital images also allow the area covered by encrusting organisms to be calculated from digital photographs with analysis software, such as Image J (Lirman et al.
2006), which is generally the most accurate method for determining percent cover (Meese and Tomich 1992). Other methods, such as stereophotography, have been successfully used for obtaining measurements from photography (Christie 1980, Lundalv and Christie 1986),
[7] but unless three-dimensional measurements are required, analysis from a single digital photograph is a simpler technique. Aside from the high initial investment, photography also has the disadvantage that it can be difficult to impossible in areas of high algal overgrowth
(Bullimore and Hiscock 2001). Surveys based around photography offer a good compromise between time spent in the water and accuracy of data.
With its increased affordability, researchers are turning to underwater video more frequently for subtidal censuses (Carleton and Done 1995, Munro 2001, Lirman et al 2006).
Like photography, it permits large areas to be surveyed while expending relatively little bottom time, but it also requires large amounts of time post-dive for analysis (Leonard and
Clark 1993). Another potential disadvantage to video is that it can be difficult to correctly locate and identify small cryptic organisms (Leonard and Clark 1993) or organisms in complex habitats (Tremblay et al. 2005). Identification down to the species level can also be difficult for morphologically similar animals, such as corals (Ninio et al. 2003). One advantage that video shares with photography is that divers do not need to be familiar with the organisms being censused (Munro 2001), which allows for a broader pool of assistants to be employed in data collection. Based on the potential difficulty of identifying small organisms, video surveys seem to be best suited for studies on larger mobile invertebrates or to quantify general characteristics of a habitat.
Survey Site and Potential Threats
Frenchman Bay is located in Downeast Maine just East of Mount Desert Island and the bulk of Acadia National Park. There was a strong fishery for Atlantic cod (Gadus morhua) and other gadids in the bay well into the 19th century (Alexander et al. 2009). More
[8] recently, the bay has been used extensively for American lobster (Homarus americanus) fishing along with a number of other fisheries. In the 1990’s the green sea urchin
(Strongylocentrotus droebachiensis) fishery began to grow throughout Downeast Maine, including Frenchman Bay, but by 2001 the fishery had crashed and landings statewide were less than a quarter of what they had been (Hunter et al. 2005, Edward Monat, 2011, Pers.
Comm.). During the same period, draggers in the Bay began harvesting orange-footed sea cucumbers (Cucumaria frondosa) and blue mussels (Mytulis edulis) (Edward Monat, 2011,
Pers. Comm.). Currently, Frenchman Bay is the site of the bulk of the fishing effort for orange-footed sea cucumber for the whole Gulf of Maine (Chen et al. 2005). Additionally, clam and mussel harvesters from around the state have converged on upper Frenchman Bay, because it is one of the few locations in the state that is not closed to harvesting because of red tide events during midsummer (Trotter, 2009). All of these activities have presumably had a range of effects on the ecology of the bay, whether through the direct removal of organisms from the water or the modification of habitat with fishing gear. Though there is some evidence to indicate a shift in ecology of the bay as a result of fisheries and other human activies (Edward Monat, 2011, Pers. Comm.), there is no solid data to describe what exactly has changed. While it would be most helpful to have data from before these intensive fisheries, it is still important to gather data now to understand any future changes that may arise from the fishing effort in the bay.
Aside from commercial fishing there are global and regional scale issues that could have powerful impacts on the ecology of Frenchman Bay. Current predictions estimate a global temperature rise of 0.3° C per decade over the next hundred years, which has the potential to cause significant shifts in marine ecosystems (IPCC 2007). Another component
[9] of climate change that is often overlooked is ocean acidification; over the next hundred years ocean pH is predicted to drop by 0.3 to 0.4 units, an increase of 100 to 150% in H+ concentration (Orr et al. 2005). A decrease in pH could have profound effects on the phytoplankton and zooplankton that form the base of the marine food chain, as well as larger organisms such as shellfish and snails (Orr et al. 2005). Although it would have been ideal to have data starting before the industrial revolution, it is essential that we gather long-term population data to understand the impacts that anthropogenic climate change is having on the ocean.
In addition to climate change, the benthic ecology of Frenchman Bay also faces the potential arrival of several marine invasive species. The Asian shore crab, Hemigrapsus sanguineus, is spreading throughout Southern Maine, but as of several years ago only a single specimen has been observed as far East as Frenchman Bay (Delaney et al. 2007). They are of particular ecological concern because of their potential to outcompete other crabs (Ahl and Moss 1999, Jensen et al. 2002). Asian shore crabs are found mainly in the intertidal, but
Codium fragile, an invasive green algae, is quite common subtidally from the New
Hampshire border to Blue Hill Bay, just to the west of Frenchman Bay, though no specimens have been found in Frenchman Bay (Chris Petersen pers. comm.). In the regions farther south where it has been introduced, Codium fragile has disrupted shellfish communities and outcompeted native kelp species (Hart 2005). Given the apparently imminent arrival of these species it is important to establish baseline data on the community as it exists currently, so that we can understand the changes that they may bring.
[10]
LP
Bi
Figure 1: Map of MDI and Frenchman Bay. Black crosses represent the survey sites. Bi=Bar Island. LP=Long Porcupine. Source: USGS. Methods
Rocky subtidal communities were surveyed at two survey sites located along the
Porcupine Islands, a chain that runs East-West across Frenchman Bay, Maine, USA (Figure
1). The sites off of Bar Island and Long Porcupine Island are at a depth of 12-15 meters and the bottom consists of a series of rocky ledges rising out of the mud and gravel. These locations were chosen based on ease of access with a small rigid inflatable combined with recommendations from a local commercial diver.
[11]
Initial random surveys were carried out at two sites, and a third site is scheduled for inclusion in the survey when the permanent plots are added. The random survey consisted of photographing 20-30 randomly placed 0.1 m2 quadrats per site and later counting all invertebrates and algae in each image. Half the quadrats were placed on horizontal substrates and the other half were on vertical surfaces to better understand the differences between the two habitat types. Horizontal surfaces were anything with a slope of less than 30 degrees and vertical surfaces had a slope of at least 60 degrees. A quadrapod built from aluminum angle stock along with a Sealife DC800 camera with a single strobe was used for the photography.
Examples of the photographs are given in figures 2 and 3.
For the permanent survey, six to eight 0.1 m2 quadrats will be marked and photographed using the same photographic techniques. The quadrats will be randomly placed on rock ledges, with half placed on vertical surfaces and half on horizontal. Originally, the quadrats were planned to be 0.25 m2 based on the work of Witman (1985) and Lundalv and
Christie (1986), but after testing with several different quadrat sizes it was decided that 0.105 m2 (30cm x 35cm) would allow for better identification. The quadrats will be marked with two brightly painted 316-grade stainless steel rods at the diagonally opposite corners of the quadrat. The marker rods will be epoxied into holes made in the rock with a pneumatic drill.
The aluminum quadropod built for the random survey will align with the site markers to ensure exact positioning each time the survey is conducted.
Four 10 m x 1 m transects will be swum for each site to quantify abundance of larger mobile invertebrates, such as lobsters (Homarus americanus), rock crabs (Cancer irroratus), and jonah crabs (Cancer borealis). In order to properly represent the lobster population, the transects will be swum in midsummer, when the
[12] lobsters are inshore for spawning. Each end of the POLY Polysiphonia sp. Filamentous red algae transect lines will be marked with a stainless steel DIDA Didemnum sp. White encrusting tunichate rod fastened by epoxy, and a line will be strung HALP Halichondria panicea between them each time the survey is conducted. Crumb of bread sponge
COR Coralline crust The transects will be placed randomly within (Lithothamnion glaciale/Phymatolithon “edge” areas that provide good habitat for lobsters rugolosum) MUD Mud and crabs. The combination of the transects and DEB Debris/dead algae etc. photoquadrats is based on a long-term monitoring ROCK Bare rock program in Massachusetts (Sebens et al. 1997), but HYDR Hydroids in family Campanulariidae with a larger quadrat size to better represent larger MICROP Microciona prolifera invertebrates. Red beard sponge
Once the permanent plots are established, METR Metridium senile Frilled anemone the following protocol will be used. The divers will HYDRA Hydractinia echinata Snail fur begin the dive by locating the transect endpoints CHOND Chondrus crispus Irish sea moss and stringing the transect line. In order to prevent FICUS Suberites ficus Fig sponge bias in the transect counts from the presence of the ERBRYZ Erect bryozoans. divers, they will then locate and photograph the MYSTBRYZ Unidentified bryozoan. Orange/red, Erect and marked quadrats. After all of the quadrats have encrusting forms. been photographed, the divers will swim the transects Table 1: Categories used for the estimation of percent cover. with a 1 meter length of PVC to delineate the boundaries of the transect. Any large mobile invertebrates with more than 50% of their body inside the transect will be counted.
The photographs were uploaded to a computer and analyzed in two ways. All individual invertebrates in the photographs large enough to be identified were counted and
[13]
Measured. Invertebrates are considered solitary if they grow independently of others of their own species. Percent cover of algae and sessile invertebrates was then estimated by projecting a network of random points across every photograph using the software Photogrid.
Due to difficulties with precise identification from the pictures, some organisms were lumped into functional groups (Table 1), similar to Sebens (1986). Invertebrates were identified using
Pollock (1998) and algae was keyed out with Villalard-Bohnsack (2003). These data will allow for comparisons not only between sites and between habitat aspects, but also to analyze the variability on a microscale between similar habitats within a site.
[14]
Figure 2: Photograph of a rock wall from Bar Island. The plot is primarily dominated by Polysiphonia sp.and there are two scarlet psoluses in the bottom center.
Figure 3: Photograph of horizontal substrate at Long Porcupine. A clump of campanularid hydroids fills the top left, and northern sea star (Asterias rubens) and frilled anemone (Metridium senile) occupy the lower right.
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Results
Overview
The two sites were quite similar in Scientific Name Common Name terms of overall bottom composition; both Myxicola infundibulum Slime worm locations consist of a series of ledges rising Psolus fabricii Scarlet psolus out of a flat muddy bottom, and both are Henricia sanguinolenta Blood star protected from the open ocean by the Asterias rubens Northern sea star Porcupine islands. At both locations bare Cucumaria frondosa Orange-footed sea cucumber rock was a rarity; almost all surfaces were Metridium senile Frilled anemone either covered in life or in thin layer of Pagarus acadianus Acadian hermit sediment or debris. Larger mobile organisms crab such as lobsters and crabs were occasionally Flabellina verrucosa Red-gilled nudibranch observed in the mud surrounding the ledges, Infraorder Doridacea Dorid nudibranch and no fishes were seen during the course of Order Stauromedusae Stalked jelly the study. Toad crabs (Hyas coarctatus), Aeolidia papillosa Maned nudibranch which are generally found in rocky subtidal Table 2: List of scientific and common names of areas, were too quick to be captured by the all solitary invertebrates counted in the photographs. photoquadrats, but were observed around the study area.
Throughout all the quadrats at both sites, a total of 21 species/groups were counted.
Of the species encountered, eight were mobile invertebrates, three were solitary sessile invertebrates, eight were colonial invertebrates, and three were algae. The average .105m2
[16] quadrat was 30% covered by living organisms and had a total of 4.15 species. There was a large range around those averages; the diversity of plots ranged from one species to nine species and the total coverage by life ranged from 3% to 91%.
Due to the resolution of the images, some species were lumped into general groups or higher taxonomic categories. The dorid nudibranchs observed were likely Onchidoris muricata, but there was no way to rule out the possibility that it was another common dorid such as Adalaria proxima or Acanthodoris pilosa. The situation was the same with the stalked jelly; it appeared to be Lucernaria quadricornis, but further examination was needed to differentiate it from the other Stauromedusae found in this region. Erect bryozoans and campanularid hydroids were lumped together because of the necessity of microscopy for positive species identification. The hydroids most likely consisted primarily of the genus
Obelia, but there may have been other genera present. Erect bryozoans included Caberea ellisi and Tricellaria ternate, but it is likely that other species were present.
Variation between sites
The two survey sites were very different in terms of the general community composition and dominant species. The horizontal substrates at Bar Island were dominated by the filamentous red algae Polysiphonia sp. with an average percent cover of 48.5%. This same species was also the most common encrusting organism at Long Porcupine, but at
12.5% or about a quarter of the area covered at Bar Island. On vertical quadrats, the difference was even more pronounced, with Polysiphonia sp. covering 49.4% at Bar Island and 6.2% at Long Porcupine. The algae cover between the two sites was significantly different (p<.001, Mann-Whitney U test statistic=419.5).
[17]
Aside from Polysiphonia sp., there was also a large difference in the population of filter feeders present at both sites (Figure 4). At Long Porcupine, hydroids were the second most common group in horizontal quadrats, after Polysiphonia sp., and they were found at seven out of the 14 surveyed sites. On the vertical plots at Long Porcupine, erect bryozoans were the most common encrusting organism (Figure 4), yet none were counted at Bar Island.
The percent cover of all filter feeders together was significantly higher at Long Porcupine
(Table 3) (p=.004, Mann-Whitney U test statistic=106.5). Several bryozoans were seen at
Bar Island during dives, but they either did not happen to fall inside a quadrat or they were not represented by the estimation of percent cover.
Long Porcupine had more diversity of solitary invertebrate life than Bar Island; a total of ten species compared to seven species. A major part of that difference was the lack of nudibranchs at the Bar Island site. At Long Porcupine, three species of nudibranch were observed: Flabellina verrucosa, Aeolidia papillosa, and an unidentified dorid. The only species found solely at Bar Island was the Acadian hermit crab (Pagarus acadianus). Despite the overall higher diversity at Long Porcupine, the average density of solitary invertebrates at
Bar Island was higher, at 2.7 m-2 compared with 1.9 m-2 at Long Porcupine. Overall, the two sites were quite different, particularly in terms of the amount of filter feeders and red algae.
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60 Bar Island Horizontal
Bar Island Vertical 50 Long Porcupine Horizontal
Long Porcupine Vertical 40
30 Percent Cover Percent 20
10
0
Figure 4: Percent coverage of encrusting invertebrates and algae on horizontal and vertical substrates at the two survey sites. The groups are arranged from most common to least common on vertical substrates at Bar Island to highlight differences in species composition. A more detailed description of groups/species is given in table 1, but the abbreviations are as follows. POLY=Polysiphonia sp., COR=Encrusting coralline algae, CHOND=Chondrus crispus, METR=Metridium senile, FICUS=Suberites ficus, HYDRA=Hydractinia sp., HALP=Halichondria panicea, HYDR=Campanularid Hydroids, DIDA=Didemnum sp., MYX=Myxicola infundibulum, ERBRYZ=Erect Bryozoan, MYSTBRYZ=Unidentified bryozoans. Error bars=1 SE.
Variation within sites
There was a large amount of variability among the quadrats within a given survey site. Histograms of the most densely occurring species, Polysiphonia sp. and campanularid hydroids are shown in figures five and six. One species, Myxicola infundibulum, was found
25 times in one photograph, four times in another, and then never in any of the other pictures from Bar Island. Polysiphonia sp. ranged from a coverage of 25 - 82% in the horizontal
[19] quadrats at Bar Island and from 0 - 42% in the horizontal quadrats at Long Porcupine. Out of the 30 coefficients of variation calculated for encrusting organisms and algae, only five were less than 1.0, and nine were greater than 3.0.
14
12
10
8
6 # ofOccurneces # 4
2
0 0-10 11-20 21-30 31-40 41-50 51-60 61-70 71-80 81-90 91-100 Percent Coverage
Figure 5: Histogram of Polysiphonia sp. coverage.
30
25
20
15
10 # of # Occurences
5
0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 % Coverage
Figure 6: Histogram of coverage of campanularid hydroids.
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30.0 Bar Island Horizontal Bar Island Vertical 25.0 Long Porcupine Horizontal
) 20.0 2 Long Porcupine Vertical
15.0
Density (/m Density 10.0
5.0
0.0
Figure 7: Counts of solitary invertebrates from horizontal and vertical substrates at both survey sites. Error bars equal 1 SE.
Horizontal vs. Vertical
Vertical surfaces generally had a higher density of living organisms than horizontal surfaces. The coverage of rock by sessile filter feeders was significantly greater on vertical surfaces at both sites (Table 3) (p=0.011, Mann-Whitney U test statistic=120.5). However, there was no significant difference between the coverage of algae between horizontal and vertical surfaces (p=0.762, Mann-Whitney U test statistic=232.5). The overall percentage of area covered by living organisms increased from 50.4% to 60.1% at Bar Island and from
21.45 to 36.1% at Long Porcupine. In general, solitary invertebrates were less common on vertical surfaces than horizontal with the exception of the blood star (Henricia sanguinolenta) at both sites and the slime worm (Myxicola infundibulum) at Bar Island
(Figure 7). Overall, the strongest difference between horizontal and vertical surfaces was the density of filter feeding organisms.
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% Algae % Filter Feeders % Bare rock % Mud/debris Bar Island 49.8 0.6 8.9 37.0 Horizontal Bar Island Vertical 49.5 11.1 5.9 24.4
Long Porcupine 12.9 9.5 5.3 69.0 Horizontal Long Porcupine 6.2 30.1 3.0 49.6 Vertical
Table 3: Percent coverage of filter feeders and algae at both sites in comparison with areas of rock, sediment and debris.
Species Interactions
90 In order to determine if there 80 70 were any significant correlations
60 between species observed in this 50 40 study, Spearman rank correlation 30
% Polysiphonia Polysiphonia % sp. coefficients were calculated for all 20 10 combinations of species occurring in 0 the same plot. There were a number 0 10 20 30 40 Hydroids of significant correlations,
Figure 8: Correlation comparison between percent cover of Polysiphonia sp. and campanularid hydroids. but the strongest was the positive relationship with coralline algae and bare rock (p < .01,
Spearman coefficient = .70, n = 42) and the negative relationship between Polysiphonia sp. and campanularid hydroids (p < .01, Spearman coefficient = -.65, n = 42) (Figure 8). Also
[22] interesting was the lack of a relationship between campanularid hydroids and either of the common nudibranchs, Flabellina verrucosa (p > .05, Spearman coefficient = .14, n = 42) and
Aelodia papillosa (p > .05, Spearman coefficient, n = 42). To see if the nudibranchs were responding to the presence of hydroids rather than the density, I ran a contingency table on the two variables from the Long Porcupine data set, but the relationship was not significant
(p=0.848. Pearson chi-square statistic=0.037, df=1). So, although nudibranchs were associated with their food at the level of site, there was not a smaller grained pattern of association between hydroids and their predators.
Patchy Distribution
Several species showed a tendency to appear in clumps, both inside the quadrats and elsewhere around the study site, so I tested for non-random distributions in all species.
However, of the species that had occurred frequently enough to be tested, only Asterias rubens (p = 0.988, KS statistic = 0.069) and Psolus fabricii (p = 1.0, KS statistic = .055) fit the Poisson distribution, so almost every species was non-randomly distributed. This indicates that practically all species observed appear in clumps to some degree.
Discussion
The data gathered in this study have provided insight into some population distribution patterns of benthic invertebrates in the Gulf Maine. Of particular note is the high level of variability both between survey sites and between photoquadrats within sites. The high variability highlights the importance of permanent monitoring sites for studying long- term trends in rocky subtidal environments in this region. The data also supports some
[23] findings of earlier studies on the different groups that inhabit vertical and horizontal rock surfaces.
Habitat Variation
The level of variation between the two survey sites was higher than expected given how apparently similar they are in overall habitat composition and exposure. Both sites were located on the protected North side of a chain of islands and consist of a series of ledges rising out of the flat expanses of mud. The only obvious difference between the structure of the ledges at the two sites is the aspect of the walls. At Bar Island, the ledges run North-
South, so the vertical rock faces are oriented towards the East, and at Long Porcupine, the ledges run East-West, so they are facing the North. This could affect the amount of light each wall receives, and since shading is one potential factor structuring rock wall communities
(Miller and Etter 2008), this could be a partial reason for the differences between the two sites. Another factor that could cause the different community composition between the two sites is the different flow regime. The Long Porcupine site likely has a much higher average flow from tidal currents because of the deeper water surrounding the island. This fits with the pattern of hydroids and bryozoans displaying a preference for increased flow rates (Hughes and Hugher 1986, Faucci and Boero 2000), since Long Porcupine was where the higher concentration of filter feeders was found. The differences between the two sites highlight the importance of incorporating a range of sites into a population study.
Horizontal vs. Vertical Surfaces
[24]
The results of this study support the findings of earlier studies describing the differences between the communities on horizontal and vertical surfaces (Sebens et al. 1997,
Miller and Etter 2008). Based their findings, the horizontal surfaces were expected to be dominated by algae, such as Polysiphonia sp., and vertical sites to be dominated by sessile filter feeders. In this study, sessile filter feeders, such as bryozoans and hydroids, were found in significantly greater densities on vertical surfaces than on horizontal surfaces, and at Long
Porcupine Polysiphonia sp. was found in higher abundance on horizontal surfaces (Figure 4).
This difference in community composition between microhabitats is potentially the result of several factors, with sedimentation and light intensity as the two most important (Irving and
Connell 2002, Miller and Etter 2008). Sedimentation has the capacity to smother low lying filter feeders and low levels of light can prevent algae from growing. Unfortunately data were not taken on relative light intensity, so it is impossible to say for certain to what degree the results of this study are a product differential light. However, at Bar Island, Polysiphonia sp. was about equally abundant on horizontal and vertical surfaces. The most pronounced difference between the horizontal and vertical surfaces was the increased density of filter feeders on wall surfaces.
Species Interactions
The strongest correlation between species within plots was the negative relationship between Polysiphonia sp. and the campanularid hydroids. Without additional habitat preference data on the two groups it is difficult to guess what is behind that interaction. The most likely explanation is that some environmental factor, such as sedimentation or flow rate, varies enough between the two sites and making one site more suitable for the algae and the
[25] other better for the hydroids. If one looks at the data (Figure 8) it is apparent that although the correlation is statistically significant, it is not adequately explained by a linear relationship.
A surprising result of the correlation tests was the lack of a link between the red- gilled nudibranchs (Flabellina verrucosa) and the campanularid hydroids, because F. verrucosa feeds on hydroids and was only found in plots where they were present. There were no nudibranchs present at Bar Island and no hydroids, but at Long Porcupine there were both nudibranchs and hydroids. Thus, it is possible that the nudibranchs may be responding to the hydroids on a larger scale than the photoquadrats can measure. If the nudibranchs are so widely dispersed that they only rarely appear in the plots, the correlation analysis may be unable to see the relationship between the species. Also, it is possible that the nudibranchs are merely responding to the presence of hydroids rather than the density of hydroids, in which case the contingency table test would have demonstrated a relationship, but it did not.
Dominant species
A similar study in the Southern Gulf of Maine found that rock walls with sea urchins tend to be dominated by them and cleared of any other major species (Sebens et al. 1997).
However, areas with low densities of urchins are dominated by Alcyonium sidereum,
Metridium senile, and Aplidium glabrum (Sebens et al. 1997). Both survey sites in this study were practically urchin free; no urchins were counted in the photoquadrats, and only a handful were observed during dives. Based on that, one might expect a similar species composition here if other factors are the same. However, I found only one of those species,
Metridium senile, and it was generally present in low numbers. Vertical substrates at Bar
[26]
Island and Long Porcupine were either dominated by the filamentous red algae, Polysiphonia sp., or to a lesser degree by erect bryozoans.
Alcyonium sidereum has shown a preference for high flow, exposed environments, both between survey sites and within individual rock walls (Sebens 1984). Metridium senile has also shown a preference for high flow environments through increased rates of pedal laceration (Anthony and Svane 1994). No similar habitat preference data are available for A. glabrum or Polysiphonia sp. It seems likely that the different species composition is at least in part a result of differing exposure and flow regime between the sites, because of the protected nature of the two survey sites in Frenchman Bay compared with the sites used by
Sebens et al. (1997). There are of course a number of other factors that could be in play, including the latitudinal difference between the studies.
The lack of green sea urchins (Strongylocentrotus droebachiensis) at the sites in
Frenchman Bay represents another key difference between my sites and the studies done farther South (Sebens et al. 1997, Miller and Etter 2008). Urchins have the potential to cause large changes in benthic communities through intensive (Siddon and Witman 2003), so this is an important factor when considering the long-term population trends of a site. The lack of urchins at Bar Island and Long Porcupine is likely a result of two major factors. First, both sites were located a few meters below the lower range of kelp (Laminaria longicruris,
Laminaria saccharina, Agrarum cribrosum), which is a major food source for urchins.
Second is the fact that ten years ago, the urchin fishery in Frenchman Bay collapsed, and urchin densities have been low since then (Edward Monat, 2011, Pers. Comm.)
Future directions
[27]
This study has begun the work of creating baseline population data for the rocky subtidal community in Frenchman Bay, and if the survey is replicated it may provide interesting insights into the population dynamics of marine invertebrates in this region, which appear to be at least somewhat different than at survey sites in the Southern Gulf of Maine
(Sebens et al. 1997), making it important to have long-term population data from this region as well. However, to be of most use, the study must be replicated and expanded. Due to the high variability between and within sites it is important to represent a number of different habitats in the survey. The addition of several more sites, particularly ones in more exposed areas, would help the survey provide a better overall picture of population dynamics in
Frenchman Bay. If random surveys are continued, the high variance within sites will make a high sample size necessary to detect the effects of environmental change. The establishment of permanent plots would reduce that need, because any change observed would definitely be the result of changes in the site rather than having sampled a slightly different spot. Thus, I have two suggestions for the continuation of this research project. First, establish two new sites in exposed areas. Potential locations are the South side of Burnt Porcupine Island and
South of the Bar Harbor Breakwater. Second, create permanent quadrats using the procedures outlined in the methods section of this paper. Replicating this survey in the future will help understand the effects that the large range of anthropogenic factors is having on the population dynamics of Frenchman Bay.
Acknowledgements: I’m grateful to Steve Ressel, my academic advisor, for all his advice and support over the past four years. My official project advisor, Chris Petersen, helped immensely with development of the project and with the revision of the final writeup. My unofficial project advisor, Eddie Monat, provided invaluable assistance with his local knowledge and long experience as a commercial diver. The data gathered during this study will be held by Chris Petersen and Eddie Monat. Without the help of many volunteers, this
[28] study would never have been completed. Thanks to Zach Whalen, Luka Negoita and Solomon Spiegel for assistance with diving and to Rowanna Herndon, Ryan Woofenden, Marina Garland, and Lindsey Erickson for acting as surface support. Boat usage was generously made possible by Sean Todd and Dan Dendanto of Allied Whale. Toby Stephenson provided safety oversight for vessel operation. Funding was provided by a Maine Space Grant Consortium Fellowship.
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