James E. Eckman

Closing the larval loop: linking larval ecology to the population dynamics of marine benthic invertebrates

The majority of marine benthic invertebrates exhibit a complex life cycle that includes separate planktonic larval, and bottom-dwelling juvenile and adult phases. To understand and predict changes in the spatial and temporal distributions, abundances, population growth rate, and population structure of a species with such a complex life cycle, it is necessary to understand the relative importance of the physical, chemical and biological properties and processes that affect individuals within both the planktonic and benthic phases. To accomplish this goal, it is necessary to study both phases within a common, quantitative framework defined in terms of some common currency. This can be done efficiently through construction and evaluation of a population dynamics model that describes the complete life cycle. Two forms that such a model might assume are reviewed: a stage-based, population matrix model, and a model that specifies discrete stages of the population, on the bottom and in the water column, in terms of simultaneous differential equations that may be solved in both space and time. Terms to be incorporated in each type of model can be formulated to describe the critical properties and processes that can affect populations within each stage of the life cycle. For both types of model it is shown how this might be accomplished using an idealized balanomorph barnacle as an example species. The critical properties and processes that affect the planktonic and benthic phases are reviewed. For larvae, these include benthic adult fecundity and fertilization success, growth and larval stage duration, mortality, larval behavior, dispersal by currents and turbulence, and larval settlement. It is possible to predict or estimate empirically all of the key terms that should be built into the larval and benthic components of the model. Thus, the challenge of formulating and evaluating a full life cycle model is achievable. Development and evaluation of such a model will be challenging because of the diverse processes which must be considered, and because of the disparities in the spatial and temporal scales appropriate to the benthic and planktonic larval phases. In evaluating model predictions it is critical that sampling schemes be matched to the spatial and temporal scales of model resolution.

Ecology of under story kelp environments. I. Effects of kelps on flow and particle transport near the bottom

Abstract Because of their likely ecological importance, the effects of understory kelps on fluid and paniculate transport near the bottom were assessed in waters of the San Juan Archipelago, Washington, U.S.A. Relative to more exposed rocky substrata at identical depths, bottoms beneath kelp canopies were exposed to weak fluid transport and were characterized by greater rates of deposition of particulates. A tracer particle experiment demonstrated that kelps inhibited transport of suspended particles from the overlying water column to the bottom. Thus, the higher rates of paniculate deposition beneath kelp canopies probably were caused by longer particle residence times and higher probabilities of paniculate redeposition beneath canopies and not by higher rates of particle import. These hydrodynamic effects may play important roles in the ecology of animals that inhabit understory kelp environments. We propose several potential effects of flow modification by kelps on larval recruitment and dispersal and growth of suspension-feeders.

Benthic flux of biogenic elements on the Southeastern US continental shelf: influence of pore water advective transport and benthic microalgae

In situ, paired light and dark benthic #ux chamber incubations were used to estimate the exchange of nutrients, oxygen and inorganic carbon across the sediment } water interface of the South Atlantic Bight (SAB) continental shelf. The results indicate that physically forced non-di!usive pore water transport and benthic primary production (BPP) by sea #oor microalgae exert a major in#uence on benthic exchange rates on the mid- and outer-continental shelf (depths of 14}40 m). Light #uxes to the sea #oor and sediment photosynthetic pigment distributions determined on two, widely spaced cross-shelf transects suggest that BPP may occur over 84% of the SAB continental shelf area. Microalgal gross BPP rates at all study sites averaged 400

Ecology of understory kelp environments. II, Effects of kelps on recruitment of benthic invertebrates

260 mg C m~2 d~1 between May and September 1996 while water column primary productivity averaged 682

Effects of flow speed, turbulence, and orientation on growth of juvenile bay scallops Argopecten irradians concentricus (Say)

176 mg C m~2 d~1, implying a total primary productivity for this region of approximately 1100 mg C m~2 d~1 (1.6 times the water column productivity alone). The results are also consistent with the advective transport of pore waters. Benthic #ux chambers appear to retard this exchange, a!ecting the accuracy of derived net #uxes. Given our inability to relate pore water gradients to #uxes in non-di!usive regimes and to mimic natural advective transport in intact core incubations, traditional techniques such as pore water gradient di!usion calculations or shipboard core incubations also may not provide accurate #ux estimates. Because of these limitations, fundamental questions remain concerning the

Transport of recently settled soft-shell clams (Mya arenaria L.) in laboratory flume flow

Previous studies demonstrated that understory kelps altered rates of mass and particulate transport beneath kelp canopies. This study examined the implications of these physical effects, plus shading by canopies, on the recruitment of four taxa of benthic invertebrates. Using field manipulations of understory kelp density and substratum orientation, we examined the effect of kelps on invertebrate recruitment and assessed the mechanisms involved. These mechanisms included reduced flow, increased sedimentation, reduced light intensity and reduced microalgal cover. All effects were significant to recruitment, but their relative importance varied among taxa. Two species examined, the serpulid polychaete Pseudochitinopoma occidental (Bush) and the cheilostome bryozoan Membranipora membranacea (Linnaeus) have long-lived planktonic larvae. Kelp canopies significantly affected recruitment of these two species, but in different ways. Pseudochitinopoma recruitment was reduced by sedimentation and light but was unaffected by ambient flow directly. Recruitment of Membranipora exhibited patterns consistent with that of passive transport of larvae with higher recruitment in high flow, no-kelp environments. Its recruitment was reduced by higher sedimentation beneath canopies, but showed no response to changes in light intensity. Spirorbid polychaetes have short planktonic phases of unknown durations. Kelps significantly affected their recruitment but difference in recruitment rates between kelp-covered and exposed substrata varied in direction with season. Spirorbid recruitment was reduced by light and high levels of sedimentation, but their recruitment patterns were unaffected by ambient flow directly. The cyclostome bryozoan Tubulipora sp. has a very short planktonic period (seconds to minutes) and factors affecting its recruitment were consequently difficult to decipher. It apparently responded negatively to higher light intensity and increased sedimentation. The effects of kelp canopies on recruitment depend on the complex interaction of these variables.

Effects of Flow Speed on Growth of Benthic Suspension Feeders

Abstract Experiments were conducted using controlled pipe flows to test the extent to which growth of juvenile bay scallops Argopecten irradians concentricus (Say) is affected by flow speed, turbulence, and orientation of individuals relative to flow direction. When flow speed was held within a narrow range (1.7–3.9 cm · s−1), scallop growth was unaffected by the presence and intensity of turbulence in flows. Scallops with exhalent pseudosiphonal openings (posteriors) oriented upstream grew significantly less than those with anteriors oriented upstream at identical flow speeds. Shell growth declined gradually with increasing velocity from 1.4–17.2 cm · s−1, but the extent of flow effects on growth varied significantly with orientation to the flow. Negative effects of higher flows on growth were apparent for scallops with posteriors oriented upstream, whereas in the reverse position effects of flow speed on growth were less obvious. Flume tests suggested that the unfavorable position, with posterior pointed upstream, was not often maintained at flow speeds > 9 cm · s−1, whereas the more favorable reverse orientation was maintained at all flow speeds examined. Velocity profiles measured in natural scallop beds, and in situ observations of the elevation of juvenile bay scallops above the bottom, revealed that juveniles routinely experienced current speeds ⩾ 17 cm · s−1. Thus, changes in orientation may provide a significant means of maintaining high growth rates under field conditions. The effect of orientation suggests that feeding by juvenile Argopecten is inhibited by back pressure established at the exhalent opening by external flow. Hydrodynamic scaling arguments indicate that adult scallops should be less sensitive than juveniles to back pressure, yet, paradoxically, previous evidence, coupled with our results, indicate that growth of adult Argopecten is more sensitive to current speed than is the growth of juveniles.

Relationship between duration of cyprid attachment and drag forces associated with detachment ofBalanus amphitrite cyprids

Abstract A laboratory flume was used to examine the retention of juvenile soft-shell clams (2 wk postsettlement) in sediment when exposed to flow. Clams resisted erosion until the initiation of sediment transport, after which they were rapidly advected from sections of test substrate. Comparisons between living and killed clams indicated that burrowing behavior was instrumental for maintaining position in flow ≤ the critical erosion velocity for sediment movement. The ability of low density, shallow-burrowing juvenile bivalves to avoid transport as bedload or resuspended particles is probably minimal during erosional periods, despite that fact that the position of larger, deeper living individuals may be unaffected. In energetic regimes, infaunal recruitment patterns can be dominated by hydrodynamic forces affecting juveniles.

Modelling some effects of behavior on larval settlement in a turbulent boundary layer

In separate experiments in 1991 and 1992, the sensitivities of growth rates of six species of benthic suspension feeder to flow speed were tested in a series of turbulent pipe flows. Species examined were the cheilostome bryozoan Membranipora membranacea (1991); the serpulid polychaete Pseudochitinopoma occidentalis (1991); and the barnacles Balanus glandula (1991), B. crenatus (1992), Semibalanus cariosus (1992), and Pollicipes polymerus (1992). In both experiments, animals were exposed to one of five constant, narrow ranges of speed that varied from about 2-15 cm s-1. Growth rates of Membranipora and Pseudochitinopoma in 1991 declined significantly and monotonically with increasing flow speed, despite evidence that at faster flows there were greater concentrations of suspended food available and higher particulate fluxes. In contrast, there was no detectable relationship between speed and growth of B. glandula over the same range of flow speeds in 1991. Results of the 1992 experiment indicated variability in growth responses among three species of barnacle. Growth rates of S. cariosus and P. polymerus were insensitive to flow speed, whereas growth rates of B. crenatus increased from low speeds to a maximum at an intermediate speed of about 8 cm s-1, and then tended to decrease at higher speeds. Combined results of the two experiments indicate that the growth response of animals to flow was most obviously related to the relative flow energy of the animals natural habitat. Growth rates of animals that typically experience relatively weak flows (P. occidentalis, M. membranacea, and B. crenatus) were affected significantly by flow speed, whereas growth rates of animals from comparatively high-energy environments (B. glandula, S. cariosus, and P. polymerus) were relatively insensitive to flow speed. In contrast, animal morphology and behavior were not obviously related to growth responses to flow. A key to understanding the impact of flow on a suspension-feeders growth may be the animals ability to handle and process particles that have impacted the feeding apparatus and to deliver them to the point of ingestion.

The role of kelp detritus in the growth of benthic suspension feeders in an understory kelp forest

Relationships between the duration of antennular attachment of cyprids ofBalanus amphitrite and forces associated with detachment were assessed in a small flume. For six of twelve sets of measurements obtained on separate dates between 27 January and 2 June 1989, drag forces associated with detachment were significantly and positively correlated with duration of attachment within the range 100 to 103 s. In general, the instantaneous drag force exerted on cyprids at detachment was more strongly correlated with duration of attachment than was either the maximum or mean force exerted prior to detachment. Our indirect method of estimating drag forces from shear velocities measured ≤0.5 cm from cyprids (i.e., ≤10 body lengths) probably underestimated the true association between detachment force and attachment time since the spatial coherence of shear velocities characterizing turbulent flows in the flume was not strong at these scales. The relationship between attachment time and drag force at detachment suggests that spatial and temporal variability in fluid forces in situ may contribute to stochastic variations in intensities of settlement ofB. amphitrite. Stresses sustained by the antennular adhesive of cyprids ofB. amphitrite were calculated to standardize forces to the area of antennular contact and permit comparisons with similar measurements made onSemibalanus balanoides by other investigators. Stresses sustained by the adhesive ofB. amphitrite were an order of magnitude lower than those reported for the adhesive of the larger cyprids ofS. balanoides. This difference may reflect differences in the stress sustainable by the adhesive secreted onto antennular pads when loaded purely in tension (measured previously) rather than in a combination of shear and tension (calculated here). Alternatively, there may be interspecific differences in the adhesive used for reversible attachment or in behavioral responses of exploring cyprids to strong flow.