Quantitative surveys of Garrison Bay fauna

Why mud?

Given the diversity of microhabitats present at Garrison Bay, why did we choose to focus our quantitative efforts on the fauna occurring in association with soft sediment?    

First, in terms of area, soft sediment is the primary habitat observed at Garrison Bay.

Second, due in part to logistical challenges associated with working and sampling in such an environment, quantitative data on marine invertebrate fauna found in soft sediment environments are generally less available than for rocky intertidal communities. Thus, there is a need for baseline data on richness and abundance and the physical parameters that influence the distributions of organisms.

Third, the marine fossil record is largely represented by organisms that lived in and/or were preserved in soft sediment environments. Three of the four members of Team Mud are paleontologists (the fourth has worked on mud flats, and is a paleontological sympathizer), and are interested in the use of modern sedimentary environments as analogs for interpreting paleoenvironments and paleocommunities in the geologic past.


Survey 1: faunal distributions along the primary transect

Along transect A, 263 live individuals were observed in core samples (n = 18) collected every 2 meters from the upper to lower intertidal. Different groups were identified to different taxonomic levels (e.g., families for polychaetes; species and genera for molluscs) and resulted in a faunal list of 25 taxa in total. The complete data matrix for the core samples can be found here .

A. Does taxonomic richness vary with distance from the shoreline?

Given variation in environmental parameters (e.g., density of burrows, temperature) along the primary transect, we asked whether taxonomic richness (the total number of different kinds of organisms) varied with distance from the shoreline and decreasing tidal height.

In Fig. 1 we compare the taxonomic richness of core samples to distance in meters from the shoreline. What is readily apparent is that while there is a weak positive relationship (such that richness appears to increase with increasing distance/decreasing tidal height), richness exhibits marked variability from sample to sample along the transect.

Fig.1.  Taxonomic richness along transect A

A large proportion of the observed variability in richness may be explained by the small sample sizes of live individuals recovered from each core sample. Sample sizes of live specimens ranged from 0 to 29 per core, with 11.5 as the median number of individuals recovered.  Such sample sizes are expected to produce considerable variability in richness and abundance; time contraints prevented our processing larger samples.

Due to the small number of live individuals recovered from the core samples, successive samples along the transect were pooled.  Data were pooled into three environmental bins: the upper intertidal zone, characterized by a surficial diatomaceous layer (quadrants 1-13); the muddy mid-tidal (quadrants 17-57); and the low-relief muddy low-tidal (quadrants 61-73). Binning the data in this way resulted in differences in sample sizes of live individuals among the three environmental zones. As estimates of taxonomic richness are highly sensitive to differences in sample size (i.e., larger samples are more likely to include more types of taxa), we used the rarefaction program in the statistical package PAST (http://folk.uio.no/ohammer/past/ ) to estimate the taxonomic richness of the three zones, given a sample size of 50 individuals (results presented below).

Beyond richness, we also wondered how the evenness of assemblages varied along the tidal gradient at Garrison Bay. Evenness is a measure of the distribution of abundance among taxa in a sample, such that in a very even assemblage all taxa occur in roughly the same abundance, while a very uneven assemblage is characterized by 1 or 2 taxa being numerically dominant and the rest of the taxa being relatively rare. To calculate evenness we used Hurlburt's Probability of Interspecific Encounter (PIE) which results in a value ranging from 0 (uneven) to 1 (even) and has been shown to be relatively insensitive to sample size effects (Hurlburt, 1971).

Fig. 2 shows a general trend of increasing taxonomic richness with increasing distance from the shoreline/decreasing tidal height. The error bars on the estimates of taxonomic richness show, however, that while the upper tidal and low tidal have significantly different values of taxonomic richness, the muddy mid-tidal is quite variable and the general trend is not characterized by discrete boundaries.

Fig. 2.  Changes in species richness and evenness over the primary transect


B. Does the composition of the fauna vary with distance from the shoreline?


The two constituents of the soft sediment environment at Garrison Bay that make up the greatest propotion of the taxonomic richness are polychaetes (45%) and bivalve molluscs (20%).  These are also the most abundant taxa, comprising the majority of individuals sampled along the primary transect (58.6% and 32.7%, respectively).

The relative abundance of polychaetes and bivalve molluscs varied with distance from the shoreline/decreasing tidal height.  Fig. 3 shows that the proportional abundance of polychaetes declines linearly across the three environmental zones with increasing distance from the shoreline/decreasing tidal height. The proportional abundance of bivalve molluscs climbs from relatively few individuals present in the upper intertidal to a fairly constant proportion in the muddy middle and lower intertidal habitats.  Other taxa also increase in abundance in the lower intertidal zone, including foraminifera and copepods which were not present in zones that experience longer durations of exposure during low tides.



Fig. 3.  Abundance of polychaetes, bivalves, and other taxa along the primary transect
These results were explored further by examining the raw abundance of four taxa which occur over much of the length of the primary transect. The four taxa include capitellids and glycerids (polychaetes) and Gemma gemma and Macoma nasuta (bivalve molluscs).  In Fig. 4, two polychaete families, while offset in their abundance peaks, occur in greater numbers in the mid to upper intertidal and then decline with increasing distance from the shoreline/decreasing tidal height. The bivalve taxa examined are largely absent from the upper intertidal diatomaceous zone, but occur at roughly constant abundance over the muddy mid to low intertidal.

Fig. 4.  Changes in abundance for four focal taxa over the primary transect


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Survey 2: Surface frequencies of anemones, gapers, & egg masses

While the primary transect provides a good estimate of the relative densities of infaunal and shallow epifaunal taxa, several taxa were observed at Garrison Bay that had a more broadly spaced distribution, and were not sampled well on the transect.  These included the gaper clam, Tresus capax , whose large siphons and burrows dot the surface of the mid to lower intertidal; the anemone, Metridium, which occurs on scattered shells in the lower intertidal; and egg masses of the bubble snails (Melanochlamys diomedea and Haminoea vesicula) that are visible on a considerable number of emergent surfaces (dead shells and branches) in the lower intertidal.  Egg masses were present along our primary transect for only 1 or 2 of the half meter quadrats.

To gather data on the frequency of these four components of the Garrison Bay fauna, the four of us (with the help of two intertidal liaisons ) walked two lateral transects along the lower-mid intertidal and lower intertidal, transects C and B, respectively.  These lateral transects covered approximately 50 m each.

Fig. 5 presents data for the lower intertidal (transect B) and lower-mid intertidal (transect C). An interesting trend emerges in which the abundance of all four groups examined is greater slightly closer to the shoreline and at slightly higher tidal heights. While the transects were relatively closely spaced in the intertidal, we observed a significant difference in the abundance of these components (e.g., a decline in the abundance of Tresus capax siphons/burrows from 113 to 35!). Furthermore, our own qualitative observations at Garrison Bay suggest that all four are rare to absent in the upper intertidal. As such, the occurrence and density of these faunal components seems to be relatively constrained to a swath of the lower-mid intertidal.


Fig. 5.  Distributions of four focal taxa along lateral transects B and C

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Survey 3: Shell - No Shell Comparisons

The dead shells of bivalve molluscs provide hard substrate 'islands' in a soft sediment 'sea' at Garrison Bay.  For example, organisms such as the sea anemone Metridium sp. are able to persist within the muddy lower intertidal by attaching by the pedal disk to disarticulated shells and shell fragments, while projecting the tentacles above the mud surface. Beyond their role in providing hard substrate, dead shells may offer protection from abiotic stresses, by acting as moisture catchments during low tides and serving as a buffer to desiccation, and from biotic stresses by providing protection from potential predators. If dead shells play these hypothesized roles, then we might expect differences in taxonomic richness, abundance, or faunal composition among samples of samples of dead shells containing sediment versus samples of nearby sediment.

To evaluate potential differences between faunal assemblages associated with shells and those found only in mud, samples of 5 shells and 5 scoops of surficial sediment were gathered every 2 meters along transect A - A'. Due to time constraints only preliminary data was gathered. These results combined with qualitative observations made during our four half-days of fieldwork are presented here.

Data were gathered on the occurrence and abundance of taxa in Shell-No Shell Comparison samples gathered from quadrants 45 & 49 (mid intertidal), and 65 (lower intertidal).  Sample sizes of live individuals were too small to test for differences in richness and abundance (median sample size = 18.5 individuals); however, preliminary results suggest that samples from shells have slightly higher taxonomic richness. The total number of live individuals in two of the three shell samples was greater than in their sediment counterparts; more data are needed for this comparison.


Hesionid polychaetes found in association with dead molluscan
valves in the surficial sediment at Garrison Bay

Do differences in taxonomic composition exist between fauna associated with dead shells and those not?

Our qualitative observations suggest the answer is "yes."  As mentioned, the occurrence of anemones at Garrison Bay is facilitated in large part by the occurrence of dead shells. Furthermore, during our fieldwork, picking up dead shells of Clinocardium nuttallii often revealed several hesionid polychaetes, found in association with the sediment/shell interface. These polychaetes are frequently found in association with large dead shells but were relatively rare in the core samples analyzed.  With further study, other taxa may turn out to have a preference for dead shell microhabitats. For example, the only occurrence of cirratulid polychaetes at the site involved the occurrence of 15+ individuals in five sediment-filled shells from quadrant 45. The gastropod Margarites sp. also occurred in greater densities in the shells sampled from quadrants 49 and 65.

Future work involving shell-no shell comparisons could prove fruitful in characterizing the role of different microhabitats in structuring the distribution and abundance of taxa in primarily soft sediment intertidal environments.

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