Raymond E. Grizzle

Historical ecology with real numbers: past and present extent and biomass of an imperilled estuarine habitat

Historic baselines are important in developing our understanding of ecosystems in the face of rapid global change. While a number of studies have sought to determine changes in extent of exploited habitats over historic timescales, few have quantified such changes prior to late twentieth century baselines. Here, we present, to our knowledge, the first ever large-scale quantitative assessment of the extent and biomass of marine habitat-forming species over a 100-year time frame. We examined records of wild native oyster abundance in the United States from a historic, yet already exploited, baseline between 1878 and 1935 (predominantly 1885–1915), and a current baseline between 1968 and 2010 (predominantly 2000–2010). We quantified the extent of oyster grounds in 39 estuaries historically and 51 estuaries from recent times. Data from 24 estuaries allowed comparison of historic to present extent and biomass. We found evidence for a 64 per cent decline in the spatial extent of oyster habitat and an 88 per cent decline in oyster biomass over time. The difference between these two numbers illustrates that current areal extent measures may be masking significant loss of habitat through degradation.

Hydrodynamically induced synchronous waving of seagrasses: ‘monami’ and its possible effects on larval mussel settlement

Underwater observations of eelgrass (Zostera marina L.) beds at the mouth of the Jordan River, Maine, USA, indicated that the eelgrass blades gently undulated with low-amplitude movements under low current speeds. When the above-canopy speeds exceeded 10 cm s−1, dramatic large-amplitude waving of many blades in synchrony occurred. The eelgrass waving caused wide variability in horizontal water current speeds measured above the canopy. During three summers (1986, 1989, 1990), the blade tips (distal 30 cm) had an average of about 3 × more recently settled (plantigrade) blue mussels (Mytilus edulis) compared to regions lower (30–90 cm) on the blade. Because waving of seagrass blades results in the blade tips moving through much more of the water column than lower regions of the blade and in enhanced turbulent mixing above the plant canopy, we hypothesize that such movements increase the likelihood of blade encounter with mussel larvae, and explain enhanced mussel abundances on blade tips. We further hypothesize that the enhanced mixing may direct larvae into seagrass beds generally. Large-amplitude, synchronous waving of terrestrial grasses has been termed ‘honami,’ (Japanese: ho = cereal; nami = wave) and has been shown to dramatically alter aerodynamical conditions within and above the grass canopy. We suggest that ‘monami’ (mo = aquatic plant) is important in coastal hydrodynamics and has major implications for larval settlement and recruitment.

Growth responses of suspension-feeding bivalve molluscs to changes in water flow: differences between siphonate and nonsiphonate taxa

Abstract Experiments were conducted in a multiple-flume apparatus to determine the effects of water flow (current speed) on individual growth of the infaunal siphonate Mercenaria mercenaria (hard clam) and the epibenthic nonsiphonate Crassostrea virginica (eastern oyster). During summer 1990 there were no significant differences in shell growth rate or final soft tissue weights of either species over a 24-day period for mean flow treatment levels of ≈2, 4 and 8 cm s −1 mid-depth (≈ffree-stream) speeds. The overall trends were a positive relation between growth and flow speed for clams, and a negative relation for oysters. A 28-day experiment in summer 1991 tested for the effects of four flow speed levels: 0, 1, 2 and 4 cm s −1 . There were substantial and marginally significant differences in shell growth rates for oysters (ANOVA, P = 0.042) and clams (ANOVA, P = 0.063). Growth response patterns were different for the two taxa, with clams showing a consistent positive relation between flow speed and growth (maximal growth at 2 to 4 cm s −1 ), and oysters showing maximal growth (average three-fold increase over other flow speeds) at 1 cm s −1 with decreased growth at 0 cm s −1 and > 1 cm s −1 . We hypothesize that the differences in growth response patterns may be explained by differences in inhalant pumping speeds, with maximal individual growth expected for suspension-feeding bivalves at ambient flow speeds that approximate the inhalant pumping speed. Because siphonate taxa typically have greater inhalant pumping speeds, we predict that ambient flow speeds optimal for individual growth will generally be several cm s −1 greater than those for nonsiphonate taxa.

A POPULATION DYNAMICS MODEL OF THE HARD CLAM, MERCENARIA MERCENARIA: DEVELOPMENT OF THE AGE- AND LENGTH-FREQUENCY STRUCTURE OF THE POPULATION

Abstract An individual-based model was developed to simulate growth of the hard clam, Mercenaria mercenaria, in response to temperature, salinity and food supply conditions. Unique characteristics of the model are that: (1) length and tissue weight are related only by condition index, so that weight, up to a point, can vary independently of length, and (2) age is decoupled from length. Tissue weight changes result from the difference in assimilation and respiration. Changes in hard clam condition are determined from a standard length-weight relationship for average hard clam growth. Changes in hard clam length (growth) occur only when condition index is greater than zero, which happens when excess weight for a given length is attained. No change in length occurs if condition index is zero (mean case) or negative (less weight than expected at a given length). This model structure resolves limitations that accompany models used to simulate the growth and development of shellfish populations. The length-frequency distribution for a cohort was developed from the individual-based model through simulation of a suite of genotypes with varying physiological capabilities. Hard clam populations were then formed by the yearly concatenation of cohorts with partially independent trajectories that are produced by cohort- and population-based processes. Development and verification of the hard clam model was done using long-term data sets from Great South Bay, New York that have been collected by the Town of Islip, New York. The ability to separately track length and age in the simulations allowed derivation of a general mathematical relationship for describing age-length relationships in hard clam populations. The mathematical relationship, which is based on a twisted bivariate Gaussian distribution, reproduces the features of age-length distributions observed for hard clam populations. The parameters obtained from fitting the twisted bivariate Gaussian to simulated hard clam length-frequency distributions obtained for varying conditions yield insight into the growth and mortality processes and population-dependent processes, compensatory and otherwise, that structured the population. This in turn provides a basis for development of theoretical models of population age-length compositions. The twisted bivariate Gaussian also offers the possibility of rapidly and inexpensively developing age-length keys, used to convert length-based data to age-based data, by permitting a relatively few known age-length pairs to be expanded into the full age- and length-frequency structure of the population.

A NEW IN SITU METHOD FOR MEASURING SESTON UPTAKE BY SUSPENSION-FEEDING BIVALVE MOLLUSCS

Abstract The most commonly used methods for measuring the amount of seston removed from the water column (uptake) by populations of suspension-feeding bivalve molluscs involve taking discrete water samples followed by laboratory analyses. Here we describe a new method based on in situ fluorometry that provides rapid measurement of seston removal rates. The new system is comprised of two identical units, each consisting of an in situ fluorometer, data logger and peristaltic pump with plastic tube attached to a deployment device. The deployment device allows precise placement of the fluorometer probe and intake end of the plastic tube so that in situ fluorescence (chlorophyll a) can be measured and water can be sampled for seston analyses in the laboratory from the same height. The typical setup involves placing one unit upstream and the other downstream of the study area and sampling the water at periodic intervals. Changes in seston concentration are revealed in the field by the fluorometers, and the sampled water can be analyzed in the laboratory for various seston parameters. Comparisons of the in situ data with data from laboratory analyses of pumped water samples were made for three species at four study sites: the eastern oyster (Crassostrea virginica), hard clam (Mercenaria mercenaria), and blue mussel (Mytilus edulis). Comparisons of measured upstream versus downstream seston concentrations indicated significant (t-tests, P < 0.05) differences (uptake) for six of eight trials based on in situ fluorometry, but only marginally significant (P < 0.10) differences at two of the four trials using laboratory chlorophyll a measurements. These data demonstrate that compared with sampling methods requiring laboratory analyses, the new in situ method provides much more rapid quantitative assessments and may provide more accurate estimates.

Chapter 8 Physiological ecology of Mercenaria mercenaria

Publisher Summary This chapter discusses the physiological ecology of Mercenaria mercenaria with emphasis on feeding, nutrition, growth, and production. It also includes a review of whole-organism behavior relevant to feeding. A consideration of energy flow through the organism provides the organizing framework. The major terms that deal with energetic are also introduced in the chapter. The various anatomical structures and associated physiological processes are considered in the chapter. The aspects of energy acquisition, which include feeding and various post-ingestion processes, are also reviewed. Energy expenditures such as biodeposition, excretion, and other metabolic “costs” such as respiration are described in the chapter. The vast majority of the research on hard clams is concerned with growth and factors that influence it. The extrinsic (environmental) factors affecting growth are also emphasized. The feeding responses largely explain the patterns of individual growth responses. The chapter also reviews the whole-organism behavior and provides a fluid mechanical perspective on the feeding environment experienced by clams in nature. Feeding occurs close to the bottom, well within the benthic boundary layer, because of the hard clams infaunal life habit and the relatively short length of its siphons.

Bottom Habitat Mapping Using Towed Underwater Videography: Subtidal Oyster Reefs as an Example Application

Abstract Towed underwater video has become a widely used method for bottom habitat mapping in coastal waters, but very little has been published on this relatively new and effective approach. We use a case study on two oyster reefs to illustrate the pros and cons of towed video, visualization techniques, and future research topics. Towed video is deployed in similar fashion to single-beam sonars, yielding narrow swaths of video imagery that are recorded concurrently with global positioning system (GPS) data for georeferencing. The major advantages over acoustic (sonar) methods are that image processing and interpretation are relatively simple, and there is little or no need for subsequent ground-truthing. The system used in the present study consists of an underwater black and white camera mounted on a steel frame, differential GPS unit, and digital video camera for recording. It was assembled from off-the-shelf items, and total cost was approximately

Current status of coral reefs in the United Arab Emirates: Distribution, extent, and community structure with implications for management

3500 (2006 US

EFFECTS OF THE FISHERY ON THE NORTHERN QUAHOG (¼HARD CLAM, MERCENARIA MERCENARIA L.) POPULATION IN GREAT SOUTH BAY, NEW YORK: A MODELING STUDY

). The imagery from both study reefs was of sufficient quality to allow classification of the surveyed bottom into three categories: nonreef, low-density shell, and high-density shell. Some reef characteristics such as the amount of vertical relief were easily discernable and showed substantial differences between the two reefs. Reef bottom areal coverages determined from the video imagery compared well with recent previous studies on the two reefs using other methods. Water clarity limitations represent the major obstacle to widespread use of video for routine mapping of oyster reefs. Turbidity–image quality relations remain to be quantified.

Restoring Ecological Functions and Increasing Community Awareness of an Urban Tidal Pond Using Blue Mussels

Coral reefs of the United Arab Emirates were once extensive, but have declined dramatically in recent decades. Marine management and policy have been hampered by outdated and inaccurate habitat maps and habitat quality information. We combined existing recent datasets with our newly mapped coral habitats to provide a current assessment of nation-wide extent, and performed quantitative surveys of communities at 23 sites to assess coral cover and composition. Over 132 km(2) of coral habitat was mapped, averaging 28.6 ± 3.8% live coral cover at surveyed sites. In the Arabian Gulf low cover, low richness Porites dominated communities characterized western Abu Dhabi, while reefs northeast of Abu Dhabi city generally contained higher richness and cover, and were dominated by merulinids (formerly faviids). Distinct communities occur in the Sea of Oman, where cover and richness were low. We provide management recommendations to enhance conservation of vulnerable coral reefs in the UAE.