The abundance and distribution of organisms within a rocky shore habitat

2.0 Abstract

The summary should include one or 2 sentences regarding what your study was about. The next three to four sentences should describe generally your methods and results. A final sentence may be used to indicate the broad implications of your work.

Rocky intertidal zone is defined by the tides and the presence of hard surfaces but the types

of organisms, the number of species, and the distribution and abundance of individual

species found in particular communities depend on the physical aspects of shores, the

supply of resources, the biological interactions among the species present. While rocky

intertidal shores often display a marked vertical zonation of fauna and flora associated with

the rise and fall of the tides, various physical factors and biological factors can lead to

complex patterns of distribution and abundance. This study used transect lines to examine

two rocky intertidal habitats. One site was exposed with hard substrate and the other site

was sheltered with soft substrate. The results showed that the sheltered site with its soft

substrate had overall higher abundance and distribution when compared with the exposed

site.

3. Introduction

The rocky intertidal zone is among the most physically harsh environments on earth. Marine

invertebrates and algae living in this habitat are pounded by waves and exposed to thermal

extremes during low tide periods (Denny and Wethey, 2001). Additionally, they must deal

with strong selective pressures related to predation and competition for space (Connell,

1961). As a result, the steep physical gradient and spatially condensed community has made

the rocky intertidal zone an ideal “natural laboratory” to study the coupled role of physical

and biological factors in determining the abundance and distribution of organisms in nature

(Connell, 1961; Paine,1994).

Marine biologists and others divide the intertidal region into three zones (low, middle, and

high), based on the overall average exposure of the zone. The low intertidal zone, which

borders on the shallow subtidal zone, is only exposed to air at the lowest of low tides and is

primarily marine in character. The mid intertidal zone is regularly exposed and submerged

by average tides. The high intertidal zone is only covered by the highest of the high tides,

and spends much of its time as terrestrial habitat. The high intertidal zone borders on the

swash zone (the region above the highest still-tide level, but which receives wave splash).

On shores exposed to heavy wave action, the intertidal zone will be influenced by waves, as

the spray from breaking waves will extend the intertidal. (Batham, 1956)

Intertidal habitats can be characterized as having either hard or soft bottoms substrates.

Rocky intertidal communities occur on rocky shores, such as headlands. Soft-sediment

habitats include sandy beaches, and intertidal wetlands. These habitats differ in levels of

abiotic, environmental factors. Rocky shores tend to have higher wave action, requiring

adaptations allowing the inhabitants to cling tightly to the rocks. Soft-bottom habitats are

generally protected from large waves. Intertidal organisms are exposed to both wave and

tidally generated water movement. In open-coast habitats subject to oceanic swell and

wind-driven waves, wave forces dominate the water movement patterns experienced by

intertidal organisms. In protected bays and estuaries, incoming and outgoing tides typically

generate long, spatially and temporally predictable periods of unidirectional flow. Tidally

generated hydrodynamic forces typically differ quantitatively and qualitatively from wave-

generated forces. Tidally generated forces are generally much smaller than wave-generated

forces, and acceleration forces are dramatically less in tidal flows. (Paine, 1994)

The tidal flows experienced by bay and estuarine organisms are largely predictable when in

contrast to the bashing wave flows seen by open coast organisms. The hydrodynamic forces

generated in exposed open-coast shoreline habitats when oceanic swell hits shorelines can

be enormous. These high water velocities and expose intertidal organisms to drag, lift, and

acceleration forces. This often precludes organisms from inhabiting these highly disturbed

habitats or limiting their inhabitants to rapidly colonizing species that are specifically

adapted to live in unstable habitats (Somero, 2002). On wave-exposed rocky shorelines, the

major adaptive challenge is typically not shifting substrate, but rather the intense force of

waves hitting the shore. Strong attachments, streamlined morphology, and living in groups

in which neighbours buffer one another from wave stresses are typical solutions to the

problems of living on wave-exposed rocky shorelines (Bertness, 2007).

Not all effects of water movement are negative. Water movement over shorelines is crucial

in delivering food and nutrients to intertidal organisms, and plays a major role in the

dispersal of the gametes and propagules of many intertidal inhabitants during tidal

immersion. Algal growth is also enhanced by high rates of flow, due to increased gas

exchange efficiency (Bertness, 2007).The food supply to intertidal organisms is subsidized by

materials carried in seawater, including photosynthesizing phytoplankton and consumer

zooplankton. These plankton are eaten by numerous forms of filter feeders eg: mussels,

clams, barnacles, sea squirts, and polychaete worms which filter seawater in their search for

planktonic food sources. (Paine, 1974)

The ocean is also a primary source of nutrients for autotrophs, photosynthesizing producers

ranging in size from microscopic algae (e.g. benthic diatoms) to huge kelps and other

seaweeds. These intertidal producers are eaten by herbivorous grazers, such as limpets that

scrape rocks clean of their diatom layer and so on up the food web. Because of these strong

positive effects of water movement on intertidal producers, high-flow habitats typically

support more productive and diverse food webs than low-flow habitats (Bertness, 2007).

Organisms living in this zone have a highly variable and often hostile environment, and have

evolved various adaptations to cope with and even exploit these conditions. One easily

visible feature of intertidal communities is vertical zonation, where the community is

divided into distinct vertical bands of specific species going up the shore. Species ability to

cope with abiotic factors associated with emersion stress, such as desiccation determines

their upper limits, while biotic interactions e.g. competition with other species sets their

lower limits. (Connell, 1975)

The intensity of climate stressors varies with relative tide height because organisms living in

areas with higher tide heights are emersed for longer periods than those living in areas with

lower tide heights. This gradient of climate with tide height leads to patterns of intertidal

zonation, with high intertidal species being more adapted to emersion stresses than low

intertidal species. These adaptations may be behavioural (i.e. movements or actions),

morphological (i.e. characteristics of external body structure), or physiological (i.e. internal

functions of cells and organs) (Menge, 1995).

In addition to being shaped by aspects of climate, intertidal habitats, especially intertidal

zonation patterns are strongly influenced by species interactions, such as predation,

competition, facilitation, and indirect interactions. Competition, especially for space, is

another dominant interaction structuring intertidal communities. Space competition is

especially fierce in rocky intertidal habitats, where habitable space is limited compared to

soft-sediment habitats in which three-dimensional space is available. Mussels, although they

are tough competitors with certain species, are also good facilitators as mussel beds provide

a three-dimensional habitat to species of snails, worms, and crustaceans (Somero, 2002).

Also important are indirect interactions, Species A eats so much of Species B that predation

on Species C decreases and Species C increases in number. Thus, Species A indirectly

benefits Species C. Pathways of indirect interactions can include all other forms of species

interactions. (Menge, 1995)

This study examines the abundance and distribution patterns of several keystone species, in

relation to elevation, hard and soft substrates within a rocky shore habitat.

4.0 Methods and Materials

2.1. Study sites

The field work was carried out on the 06/09/2010, on Moturaki Island Located

approximately 500 km east of Mount Maunganui in bay of plenty of the north island of New

Zealand. The Bay of Plenty has prevailing westerly winds with subtropical waters and an

average water temperature of 17°C (NIWA website). Site A was located on the exposed

northern end of the island which consisted of a rocky hard substrate. Site B was a soft

substrate habitat, located at the sheltered south eastern end of the island. These two sites

were chosen specifically to represent soft and hard rocky shore substrates used to test the

hypothesis of consistent patterns of abundance and distribution and in relation to

elevation.

Site A: Exposed, hard substrate.

Site B: sheltered, soft substrate

.

The area of the rocky shore habitat used in this study extended from 0 cm above sea level to

about twenty meters above the mean low water level with both sites. A Flexible 50 metre

measuring tape was used to measure the transect length. All data was recorded in the field.

Plots were sampled visually using a plastic tubular frame (200x200cm). For algae, Estimates

of coverage were obtained by visually assessing coverage assigning each taxon in each

quadrant a score ranging from 0 to 100% and adding up the total in each quadrant. Limpets

and other animals were counted in each of the quadrants. Final values were expressed as

percentages. The quadrants were alternated at each 2 metre interval and sampled non-

destructively.

(Quadrants) (Two metre spaced intervals)

Figure 4: shows a diagram of the type of transect used

Patterns of abundance and distribution were investigated by sampling the two rocky shore

habitats. At each site, One transect line was placed starting from sea level and heading in

the opposite direction. Elevation was recorded by placing staffs spirit levels along the 20

metre transect line, recording the height measurement at each quadrant (200x200 cm).

5.0 Results

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Chart Title

Metres from Sea Level

Spec

ies

Figure 1: Overall the exposed site had lower abundance and distribution when compared with the sheltered site.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Chart Title

metres from sea level

spec

ies

Figure 2: shows that the sheltered site had higher abundance and distribution when compared with the

exposed site.

6.0 Discussion

Generally the data shows an overall increase in abundance and distribution in the sheltered

site when compared with the exposed site. Suggesting that sheltered habitats are more

preferred. Macro algae was present at lower levels with both sites however in the sheltered

site it is found at slightly higher elevation. Upon Inspection of the data it is observed that

barnacles mostly found at higher elevations with both sites. In the sheltered site Mussels

were found in greater numbers at lower levels however with the exposed site a more even

distribution is observed. Periwinkles were found at higher levels with the sheltered site and

a few small dispersed patches in the exposed site. Limpet distribution was dispersed in both

sites however greater numbers were recorded in the sheltered site. Lichen was present at

higher elevation in the exposed site.

Rocky shores and solid substrates down to the lower limit of the euphotic zone provide the

main habitat for microalgae. So the observation that macro algae were only found close to

sea level is not surprising. There is an increase in microalgae cover on sheltered rocky shores

compared to exposed rocky shores. This could be because Where Wave action is low

seaweeds stay attached to substrates and don't open up new areas of shore for re-

colonization by animals such as limpets (reduced disturbance compared to exposed rocky

shores).

The sheltered site was also the more shaded and therefore damper. Shaded slopes will remain

damp for longer than those which are sunny, this may result in zones on shaded slopes

extending further up the shore. This could explain the higher elevation noticed with algae in

this site. It is generally accepted (at present) that the upward limitation of organisms on a

rocky shore is controlled by physical factors such as desiccation (Nybakken 1988).

The results of transplantation experiments that move macroalgae down the shore suggest

that growth is not inhibited by the changing physical conditions and that it is more likely

that competition and grazing at the bottom of zones controls their downward extent.

However, interactions between organisms on a rocky shore and their environment are

complex and it is likely that explanations for zonation may differ between locations. Several

studies have showen that grazing by gastropods and small crustations is a major factor in

algal zonation (Kautsky 1993).

It has been said that “The barnacle zone on rocky shores is the highest zone" (Anderson,

1994). The data seems to support this. On well-drained, sunny, intertidal rock surfaces,

barnacles are often the most numerous animals, but where surfaces are shaded or slow to

drain other sessile animals (and algae) are evermore present. On certain shores,

competition has been shown to be an important factor in bringing about the barnacle

zonation observed (Connell 1961a). Among sessile animals which filter their food from the

sea, competition is primarily for space to settle and grow. With their thick shells, they are

also well equipped to resist the damaging effects of dessication, temperature extremes and

ultraviolet radiation (Anderson, 1994). Extreme neap tide high water mark is the

approximate theoretical upper limit for barnacle growth, although it moves upward with

degrees of wave exposure as much of their food is extracted from the thin layer of water

from surge and spray (Anderson, 1994). Therefore it would make sense that barnacles with

there ability to survive in tough conditions and the need for surface area to inhabit, they

would be found in areas where other competitors wouldn’t be found. The data seems to

support this theory.

When examining the data it’s obvious that the majority of the mussels are

at the lower levels of the intertidal zone. This is most likely because

Mussels must be submerged in water in order to feed because they are

filter feeders. Their darker colored shells also probably attract a lot of

sun making them venerable to temperature stress. These characteristics

of mussels are physical limitations that prevent it from surviving too high

in the intertidal where water is most infrequent. The lower intertidal is

nice and moist, making it a desirable place for mussels to stay, except

that there are an abundance of predators living there, sea stars, snail and

crabs). This leaves the mid-intertidal as the most likely place for mussels

to survive, which is where they are usually found (Menge and Branch

2001).

In the mid-upper intertidal the presence of barnacles enhances the

settlement of mussels by providing a rough-textured surface on which

mussels show higher preference for and survivorship. When mussel

recruitment and growth rates are high the mussels actually take over and

suffocate the barnacles (Bertness, 1999). The data collected from both

sites seems to support this. As the mussel numbers decrease as elevation

increases barnacle numbers increase and vice versa. This suggests a

direct relationship in relation to competition for habitat.

There seems to be variation in the zonation of periwinkles. The distribution is not uniform

along the intertidal zone and varies from site to site. The data seems to show that in the

exposed site, the periwinkle is fewer in numbers and more distributed over elevation.

However in the sheltered site a more distinctive zonation is observed higher in elevation

with much greater numbers.

According to studies by (Little & Kitching, 1996) the distribution of periwinkles appears to

have no relation to light, tidal level, or beach slope; periwinkles can be found in clusters,

especially in crevices and tide pools, or alone. Periwinkles have been observed to remain

inactive or stationary during the majority of each tide, even when covered with water. Most

of the periwinkles were distributed near the higher Intertidal regions because the

periwinkles at the low tidal region were susceptible to being eaten by organisms such as

chitons. Periwinkles are most abundant on sheltered shores because they feed on algae. As

conditions become more exposed the algal cover in which the periwinkles feed on reduces

and become more random.

The distribution of limpets at both sites seems to be varied with slightly higher numbers at

the sheltered site. This pattern has been observed in other studies (smith 1975). The higher

abundance observed at the sheltered site is most likely because the algae in which limpets

feed on is greater in this area because the site has less sunlight and more moisture making it

more suitable for algal growth. These conditions are also the reason for limpets being able

to climb to higher elevation as seen in figure 2.

7.0 Conclusions and recommendations

The study was a good introduction into the complexity of the relationships between

organism’s and the intertidal areas they inhabit. The results have largely supported the

literature relating to rocky shore habitats and in relation to soft and hard substrates. To gain

a better understanding of the intertidal zones, future studies should include a more detailed

approach to outlying variables such as sunlight and exposure. Also a more detailed

observation of the families would help to better understand intraspecific relationships.

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