Biological Interactions and Physical Stress Determine Vertical Zonation in the Rocky Intertidal: Scientific Paper

[Created Oct. 23, 2015]

Bailey Farris, Matthew Layton, John Ripoli, Colin Thomas
Department of Marine Sciences, University of New England

ABSTRACT
Vertical zonation is an intricate part of rocky intertidal ecosystems that allow for species

to coexist in a community together. In order to determine what influences the limits of vertical zonation, a study was completed along a segment of the rocky intertidal coastline of Maine. The relationship between algae, sessile, and herbivore species can be examined to explain how competition and physical stress impact vertical zonation. Using transects and abundance measuring techniques, species in the rocky intertidal could be counted at different tidal heights and compared. While many species had overlaps in vertical zonation, there were also clear divides of where a certain species was dominating. In each scenario, either physical stress or biological interactions determined the range from which a species existed.

INTRODUCTION
The rocky intertidal is an ecosystem composed of two different substrates on the East

Coast of Maine. The two substrates are emergent substrate and boulder fields. Emergent substrates are non-mobile rock structures that are unaffected by wave drag. Boulder fields are composed of smaller mobile rocks that are moved about by wave drag. A section of the rocky intertidal can be categorized as either exposed or sheltered. An exposed section is typically steep, allowing for wave splash to travel higher up a vertical tidal distance. Sheltered sections are flat and reduce wave splash by allowing waves to roll up over the substrate.

One important study on vertical zonation was completed by Joseph Connell on the relationships on marine coasts. His study discusses the proven theory that lower limits of vertical zonation are determined by biological competition, while upper limits of vertical zonation are determined by physical stress. Connell’s study examined how the most important physical need of marine organisms is seawater (Connell, 1972). Without this coverage, many organisms won’t survive. There are many coping mechanisms adapted by organisms to deal with physical stress like desiccation. A few examples are sealing water inside an organism’s shell, retreating to wet regions, or adapting to survive water loss (Guay, 2015). Desiccation is a major physical stress that impacts the upper limit of where a species can survive. Biological competition can refer to both predation or the competition for resources between organisms. Connell explains how this is evident when a species physically can survive in an area but still isn’t found there.

The goal of this study was to investigate how vertical zonation was determined in a rocky intertidal zone. The rocky intertidal zone explored was on the Gulf of Maine. Field research was completed to test how vertical zonation was impacted by biological competition and physical stress.

MATERIALS AND METHODS

Site Description –

Organism abundance was observed and counted at the Ocean Point Beach, Boothbay Harbor, Maine 2004. Low tide was in the evening around 5:00pm with a measurement of – 0.34m. Waters got rougher throughout the afternoon, and wave splash increased over time. The site was composed of an exposed rocky intertidal with both covered and bare surfaces. Zones closer to the low tide line were composed emergent substrate, while higher inland was cobble. Subjects –

A multitude of organisms were measured throughout this experiment and broken down into different subcategories to compare abundance observations along two separate transects. Herbivores along the exposed transect included Littorina littorea (common periwinkle), Littorina obtusata (flat periwinkle), and Littorina saxatilis (rough periwinkle). Primary producers along the exposed transect were Calothrix (blue algae), Fucus vesiculosis (bladder rockweed), Fucus distichus (sea rockweed), Mastocarpus stellatus (false Irish moss), and Chondus crispus (Irish moss). Sessile organisms observed were Semibalanus balanoid (barnacle) and Mytilus edulis (blue mussel), while the predators studied were Nucella (Dogwhelk), Asterias spp (sea star), and Carcius maenas (European Green Crab).
Experimental Design –

This experiment was designed to examine patterns of fauna and algal vertical zonation, species abundance, and species composition between varying levels of exposed slopes along a section of the rocky intertidal coast of Maine. This includes exploring the impact of physical stress and competition stress on rocky intertidal organisms. Along four transects, species abundance was examined at different stations. Vertical changes in distance along the transect were compared to the distance between each station and the low tide level. Organisms were identified and quantified at each station for abundance. In this analysis, data from transects two and four were inspected. For each station there were three samples were collected, averaging out to 21 samples for transect two and 39 samples for transect four.
Measuring Abundance –

Two measuring techniques were used for data collection: a stick and string method, and a quadrant method. The stick and string method was used to collect data about the topographical profile of each transect. The quadrant method was used to count abundance of organisms along

each transect. Each quadrant was 0.25m x 0.25m in size, and three quadrants were counted at each station. Sessile organisms were counted using percent coverage, taking into account bare space. Mobile, or potentially mobile, organisms were measured by individual count. Statistical Analysis –

Once data was collected from the site, it was analyzed for comparison between the two transects. Average abundances and standard error were the major calculations completed to compare zone compositions. Standard deviation was used in order to later find standard error. Tidal height and vertical distance was analyzed for profile graphing of the transects.

RESULTS
Of the two transects compared, transect four had a steeper, more exposed slope than

transect two (Figure 1).
Calothrix was found in tidal heights ranging from 2.5m and 2.1m, with decreasing

concentrations relating to decreasing tidal height (Figure 2). The average abundance was 36.5%+25 coverage (Figure 2). Chrondus crispus inhabited tidal heights averaging 1.1m to – 0.15m, and with a positive skew towards at lower tidal heights (Figure 2). The average abundance was 28% +8 coverage (Figure 2). Fucus vesiculosis had a negative skewed abundance between 1.5m and 0.7m tidal height, peaking around 1.1m (Figure 2). The average abundance was 8%+2 coverage (Figure 2). Mastocarpus stellatus ranged from tidal heights of 1.5m to 0.2m, with distribution skewed to the left, peaking at 0.7m (Figure 2). The average abundance was 45% +5.3 coverage (Figure 2). Fucus dis. was observed ranging in tidal heights of 0.7m to -0.15m, slightly skewed to the right but with an almost normal distribution (Figure 2). The average abundance was 4.6% +2 coverage (Figure 2).

Littorina littorea was the most abundant herbivore, with the greatest abundance at 2.39m, an average abundance 1019 +490.5 individuals/m2 (Figure 3). The second most abundant herbivore was Littorina obstusata, overlapping with Littorina littorea at the 1.99m tidal height (Figure 3). The average abundance 682 +24.5 individuals/m2 (Figure 3). Some Littorina saxtilis was present in a small average abundance of >30 individuals/m2(Figure 3).

On transect four, Semibalanus balanoid had an average percent coverage of 34%+5.2 and a range from 2.79m to 1.59m (Figure 4). Mytilus edylis was found in tidal heights from 2.79m to 0.39m with a mean abundance %30 +5.25 coverage (Figure 4). On transect 2, M. edylis had a small range from 1.5m to 1.1m (Figure 6). Semibalanus balanoid ranged from 2.1m to 0.7m, overlapping Mytilus edylis (Figure 6).

In an exposed intertidal zone, Nucella was found in an average abundance 64 +35.5 individuals/m2 ranging from 1.99m to 0.39m (Figure 5). Asterias’s upper limit shared Nucella’s lower limit of 0.39m and ranged down to -0.15m tidal height (Figure 5). The average abundance of individuals of Asterias was 27 +5.20 individuals/m2 (Figure 5). In a sheltered zone, Asterias had a range from 0.2m to -0.15m, and an average abundance of 5.3 individuals/m2 (Figure 7). Carcius maenas was found in the sheltered regions from 1.5m to 0.7m with an average abundance of 16 +9.5 individuals/m2 (Figure 7).

DISCUSSION
Most of the results in this paper were congruent with the hypothesis that vertical zonation

upper limits are set by physical stress, while lower limits are set by biological interactions. Limits are different tidal heights at which a species inhabits, and the range of these tidal heights varied between transects of different steepness. Both transect four and transect two exist along an exposed rocky intertidal coastline. Transect four is more exposed though because of the

steepness of the slope (Figure 1). In comparison, transect two is a sheltered transect (Figure 1). With greater steepness, wave splash is increased up the tidal zones. Organisms on the steeper slope have to worry more about desiccation while organisms on the shallower slope need to worry about wave and current drag. A steeper slope allows for wave splash to keep higher surfaces wet throughout lower tide times without being completely submerged. Organisms in this zone need to deal with the stress of open air, and taking in the most they can from water spray. Meanwhile, organisms in the sheltered zones need to deal with the constant push and pull of strong waves breaking, pushing, and pulling. Organisms here need adaptations to keep them rooted, or suctioned, to their substrates. The effect of an exposed or sheltered transect is seen in the range of tidal heights examined.

Calothrix is a species of algae that survives best when it isn’t submerged its entire life (Johannesson et el., 2000). By this logic, Calothrix will peak in areas at and above mean tidal heights. In this study, Calothrix doesn’t exist below a mean tidal height of 2.1m, which is above the total mean tidal height of the rocky intertidal zone. According the study “Community Interactions on Marine Rocky Intertidal Zones” by Joseph Connell, proof that lower limits are determined by biological interactions occur when a species could survive at a lower tidal height, but doesn’t inhabit the extension of their full lower limit (Connell, 1972). Calothrix could survive in this exposed rocky intertidal up to the mid tide range, but instead stops in the upper intertidal zone right when Fucus vesiculosis is introduced. Fucus vesiculosis is outcompeting Calothrix for resources such as nutrients and space in the lower tidal zones.

There is distinct overlap of Fucus ves. and Fucus dis. in at 0.7m tidal height (Figure 2). Interesting, considering these are two of the same species, but different genius. This is a great example of how physical stress and competition set the boundaries of species distribution. Upper

limits are determined by physical stress and lower limits are determined by biological stress. Fucus ves. isn’t a streamlined plant and gets torn off rocks in rougher waters with more wave drag. Fucus dis. is a flat stemmed plant that needs the wave drag to pull it up towards the surface for photosynthesis. (Guay, 2015) It would make sense that these plants are distributed where they are because Fucus dis. at lower heights can have constant current pulling it around for light, and Fucus ves. can escape the constant tug of the current by being where the tide retreats from. Higher on the vertical coastline, Fucus ves. will still get sprayed by water to avoid desiccation, but will be fighting less current. Meanwhile, Fucus dis. will always be exposed to the pull of tides and currents towards and away from shore throughout the day, year round. The bladders in Fucus ves. can retain water to also help prevent desiccation (Guay, 2015). Physical stress from desiccation prevents Fucus dis. from moving father up the coastline, and Fucus ves. outcompetes the other for biological resources and space higher up.

Of the three Littorina species, Littorina littorea and Littorina obtusata had the most defined overlap of vertical zonation. At 1.99m tidal height both species coexist, and there is a point of competition between the two species. (Figure 3) Physical stress from desiccation prevents Littorina obtusata from surviving higher in the tidal zone due to its flat nature. L. obtusata can’t retain as much water in its shell during low tides as L. littorea can. Due to this, Littorina obtusata needs to inhabit a zone with less time exposed to the air and more time submerged. Competition with L. obtusata prevents Littorina littorea from over taking lower tidal zones. Both of these species share resources though for an overlapping area. Tidal height 1.99m is approximately the middle of the intertidal zone that ranges from 4.58m to 0.39m (Figure 3). This middle ground experiences an approximately equal amount of time covered by the tide and exposed to the air. Risk of desiccation might prevent L. obtusata from living farther up the zone,

but is still manageable on the lower side of the midway point. L. littorea is adapted to deal with desiccation, but is outcompeted by L. obtusata for resources. Both of these species face a partial stress from physical and biological competition, but not enough to rid them from half way down the intertidal zone. Competition keeps the L. littorea population in check, and desiccation keeps the L. obtusata population in check so neither population pushes out the other completely.

Dealing with the Littorina species it is also important to note that Littorina saxatilis was expected to be present and was not. An explanation for this is the competitive exclusion by Littorina littorea on Littorina saxatilis (Yamada and Mansour, 1987). Both species compete for the same food source, and as a larger predator L. littorea outcompetes L. saxatilis (Yamada and Mansour, 1987). This biological interaction has not only impacted the distribution of a species, but also the presence.

Chrondus crispus also inhabited the mid and lower tidal height zones, like Littorina obstusata (Figure 2). This contradicts normal trends of this species’ habitat. Chrondus crispus is a species that dominates exposed substrates because it competes so well for space due to the fact that it is a K-selected species (Guay, 2015). Chrondus crispus can exist in higher tidal ranges in an exposed transect also due to the high wave splash. For Calothrix to dominate above Chrondus crispus means that there is some factor that Calothrix outcompetes Chrondus crispus for (Guay, 2015). While this goes against the concept that the upper limit is set by physical stress, it would explain why Chrondus crispus does not survive in the upper limit where Calothrix is. A new explanation for this could be that when two species can survive in the same vertical zone without fear of physical stress, biological competition determines the divide between species. Chrondus crispus also competes with Mastrocarpus in a similar overlapping fashion. Mastrocarpus was found in zones above or equal to Chrondus crispus (Figure 2). There are two reasons for limits

set between C. crispus and Mastocarpus: freezing and space competition (Guay, 2015). In Maine during the winter months Mastocarpus can survive long exposure to cold air without suffering negative effects on growth until after three hours (Dudgeon et al., 1990). C. crispus on the other hand suffers in growth rates after initial freezing. The upper limit of C. crispus against Mastocarpus is determined by the physical stress of freezing. The lower limit of Mastocarpus is set by a biological competition. C. crispus outcompetes Mastocarpus for space because C. crispus’s holdfasts encrust the substrate and secrete toxins to prevent other species from putting down holds (Castro, 2012). In general, Chrondus crispus is a great competitor for space in an exposed rocky intertidal because of its ability to take over an area so quickly, but given space of bare rock other species will push their way into the same zone.

Between the exposed and sheltered transect, Mytilus edylis and Semibalanus balanoid both cover different zones, comparing within their own species (Figure 4)( Figure 6). While there is some overlap of zonation, there is a general trend that both species cover a larger range in the exposed zone than in the sheltered zone. Overlap in species distribution can be explained by predation of Nucella keeping both populations at a stable level where the abundance of both can thrive in their overlap (Bertness, 2001) Nucella predation on both species is a form of indirect competition between Mytilus edylis and Semibalanus balanoid. These organisms also range to higher points in vertical tide zonation for the exposed transect four. Both barnacles and mussels fight against desiccation, and exposed regions have higher wave splash (Guay, 2015). This wave splash allows for organisms to stay wet at higher points in the intertidal during low tide. Wave splash therefore helps prevent desiccation, allowing for organisms to take in a small amount of water at low tide through splash versus struggling to maintain whatever amount of water trapped

before the tide goes out. Comparing exposed versus sheltered characteristics explains how this species distribution changes in the with the same trend for both of these sessile organisms.

One type of competition is predation, and predation can be a factor in setting not just the lower limit for species distribution but also the entire rocky intertidal zonation. Predation is an example of interference competition where two species have direct contact and impact with each other (Castro, 2012). Predation is just as strong of an environmental factor as things such as wave drag (Trussell, 1994). Nucella and Carcius maenas are two predators found in the exposed intertidal and sheltered intertidal respectively. Green crabs aren’t picky with their diet, and one food source for them is Nucella. The fact that Nucella doesn’t exist in the same area as Carcius maenas can be explained by Carcius maenas outcompeting (through predation) Nucella. If Nucella was taken out of a system with Carcius maenas, Carius maenas wouldn’t be forced to follow this single food source, because Carcius maenas is adapted to eat a wide range of food (Robinson et al., 2011). Studies have show that while Carcnius maenas can survive off many different species, they thrive in feeding areas with less variety and easier prey (Robinson et al., 2011). Nucella would benefit from inhabiting areas without Carcius maenas present to avoid predation. Without Carcius maenas, Nucella can focus on surviving physical stress and finding food, instead of stressing over adapting to intense predation.

The Asterias predator was found in both exposed and sheltered rocky intertidal zones (Figure 5)( Figure 7). In both scenarios, this species existed in the lower end of the tidal heights, closer to low tide. This allows for a more constant water coverage, which Asterias needs to avoid desiccation. As discussed before by Connell’s experiment, the upper limit of vertical zonation is determined by physical stress (Connell, 1972).

Desiccation and resource competition are the two most common reasons for vertical zonation limits. Upper limits of vertical zonation are set by physical stress, such as desiccation or freezing. Lower limits are determined by biological competition for resources or predation. A new hypothesis that vertical zonation for species of equal physical competition is determined by biological interactions for both the upper and lower limit arose from examining the relationship between Calothrix and Chrondus crispus. In summary, the trends in this study suggest that physical stress and biological are the main determining factors of vertical zonation in a rocky intertidal.

LITERATURE CITED
Bertness M. 1984. Habitat and community modification by an introduced herbivorous snail

[Internet]. Brown University; [cited 2015 Oct 22]. Available from: https://elearn.une.edu/webapps/blackboard/execute/content/file?cmd=view&content_id=_12018 00_1&course_id=_36752_1

Bertness M. 2001. The ecology of Atlantic shorelines. Journal of Ecology; [cited 2015 Oct 22]. Available from: http://onlinelibrary.wiley.com/doi/10.1046/j.1365-2745.2000.00447-2.x/citedby

Castro P, Huber M. 2012. Marine Biology. (9). McGraw-Hills Publishers; [cited 2015 Oct 22].

Connell J. 1971. Community interactions on marine rocky intertidal shores [Internet]. University of California; [cited 2015 Oct 20]. Available from: https://elearn.une.edu/bbcswebdav/pid- 1201810-dt-content-rid-8716922_1/courses/22067-201602-MAR-250L- B/Connell_1972%281%29.pdf

Dudgeon S et al. 1990. Marine biology [Internet]. California State University Northridge; [cited 2015 Oct 20].
Guay D. 2015 Marine ecology 250 lecture. University of New England.

Johnnesson B, Larsvik M, Loo L, Samelsson H. 2000. Cyanobacteria [Internet]. Tjärnö Marine Biological Laboratory; [cited 2015 Oct 20]. Available from: http://www.vattenkikaren.gu.se/fakta/arter/bacteria/calothri/calospe.html

Robinson E, Smee D, Trussel G. 2011. Green crab foraging efficiency reduced by fast flows [Internet]. Texas A&M University; [cited 2015 Oct 22]. Available from: http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0021025

Trussell G. 1994. Phenotypic plasticity in an intermediate snail: the role of a common crab predator [Internet]. University of New Hampshire; [cited Fri Oct 23]. Available from: https://elearn.une.edu/webapps/blackboard/execute/content/file?cmd=view&content_id=_12018 24_1&course_id=_36752_1

Yamada S, Mansour R. 1987. Growth inhibition of native Littorina saxatilis by introduced Littorina littorea [Internet]. 187-196 (105). Available from: https://elearn.une.edu/bbcswebdav/pid-1201827-dt-content-rid-8716904_1/courses/22067- 201602-MAR-250L-B/Y amada_Mansour_1987%281%29.pdf

Leave a Reply

Your email address will not be published. Required fields are marked *