Timothy C. Granata

An approach to measurement of particle flux and sediment retention within seagrass (Posidonia oceanica) meadows

Abstract Seagrass beds have traditionally been considered to act as sinks for particles due to the reduction of flow velocities by the plant canopy. Yet, there is a paucity of measurements to confirm this role. In this work we illustrate changes in flow in the presence and absence of Posidonia oceanica using an ADV, and provide direct measures of particle trapping by the use of sediment traps. We also describe a model to estimate sediment resuspension after measuring particle flux at different distances from the bottom. Measurements of particle flux are conducted parallel to the study of structural parameters of the Posidonia meadow potentially involved in both particle trapping and retention. Data obtained on velocity profiles confirm previous findings that seagrass canopies slow down current velocities with intensities proportional to the canopy height of the plants. The projected surface area of the plants (LAI) significantly correlated with the total amount of particles trapped within the Posidonia meadow, thus indicating seagrass canopy slightly increased particle trapping in the absence of resuspension. The trapping capacity of the canopy was not linearly correlated to LAI but significantly decreased at LAI above four, thus suggesting that other factors such as bending of the leaves and particle attachment to the surface may interfere with particle free sinking within the canopy at high projected surface area. The model proposed to estimate resuspension allowed to measure the retention capacity of the P. oceanica meadow, this being up to 15 times higher compared to a barren bottom during situations of high energy (large eddies reaching the bottom). The results obtained provide direct quantitative support to seagrass beds promoting sediment accretion and demonstrate a promising avenue to provide the needed empirical support for the effect of seagrasses on depositional processes.

Concurrent high resolution bio‐optical and physical time series observations in the Sargasso Sea during the spring of 1987

The evolution of bio-optical and physical properties of the upper layer of the open ocean has been examined at time scales from a few minutes to several months using recently developed multi-variable moored systems (MVMS). Concurrent, colocated time series measurements of horizontal currents, temperature, photosynthetically available radiation, transmission of a beam of collimated light (660 nm), stimulated chlorophyll fluorescence, and dissolved oxygen concentration were made. The systems were located at eight depths in the upper 160 m of the Sargasso Sea (34°N, 70°W) and were deployed three times for a total of 9 months in 1987. The first deployment data presented here show considerably more variability than those of the latter two deployments because of the dynamic springtime shoaling of the mixed layer and the accompanying phytoplankton bloom and more mesoscale variability associated with cold core rings and warm outbreak waters associated with the Gulf Stream. These data are used to demonstrate the utility of the MVMS and indicate the importance of high-frequency, long-term sampling of bio-optical and physical variables of the upper ocean for understanding and modeling dynamical changes in bio-optical properties, primary production, and carbon fluxes of the upper ocean on time scales ranging from minutes to seasons to decades. Some phenomena observed with the systems include (1) diurnal variations in bio-optical properties, (2) springtime stratification and rapid (∼2 days and less) episodic changes in the beam attenuation coefficient and in situ chlorophyll fluorescence, and (3) advective episodes associated with warm outbreaks of Gulf Stream waters and cold core Gulf Stream rings in the vicinity of the mooring.

Seasonal variability of bio-optical and physical properties in the Sargasso Sea

The seasonal variability of bio-optical and physical properties within the upper ocean at a site in the Sargasso Sea (34°N, 70°W) has been observed using multivariable moored systems (MVMS) during a 9-month period (March through November 1987). In addition, complementary meteorological data, sea surface height (Geosat) and sea surface temperature maps, and expendable bathythermograph (XBT) and shipboard profile data (physical and bio-optical) have been utilized for interpretation. The observations during March are characteristic of late wintertime conditions of a deep isothermal layer (∼18–19°C), but with intervening periods of warming due to the advection of warm outbreak waters associated with Gulf Stream meanders. The mixed layer depth shoals from greater than 160 m to about 25 m in late March (spring transition). Phytoplankton blooms follow the mixed layer shoaling. A succession of phytoplankton populations occurs during this transitional interval. Mesoscale variability associated with cold core rings and warm outbreak waters associated with the Gulf Stream are evident at various times. The mixed layer remains near 25 m for the summer and deepens in mid-September. A relatively intense subsurface maximum in chlorophyll develops at ∼75 m following the spring transition. The maximum persists, but weakens in mid-summer. The present study clearly indicates that important processes associated with and contributing to the seasonal cycle occur on short time and space scales and that integrated data sets obtained from moorings, ships, and satellites can be used to effectively study bio-optical and physical phenomena on time scales from minutes to seasons.

Estimation of Seasonal Primary Production From Moored Optical Sensors in the Sargasso Sea

A bio-optical mooring, which was deployed for 240 days during 1987 in the western Sargasso Sea (∼35°N, 70°W), provided among other things a detailed record of the seasonal distribution of chlorophyll a fluorescence and the scalar irradiance of photosynthetically available radiation. This data base was augmented by shipboard measurements of chlorophyll a concentration, chlorophyll a fluorescence, scalar irradiance, and net photosynthetic rate during four seasonal cruises to the mooring. The combined data base clearly shows a spring bloom in the surface mixed layer associated with initial stabilization of the water column, formation of a subsurface chlorophyll maximum caused by enhanced stratification of the water column, and disappearance of this feature in the winter caused by deepening of the surface mixed layer. The moored and shipboard data were applied to a detailed description of the seasonal variability in the vertical distribution of chlorophyll a and photosynthetic rate. Specifically, photosynthetic rate at a given depth was calculated as the product of scalar irradiance, chlorophyll concentration, the specific absorption coefficient of the phytoplankton crop, and the quantum yield of photosynthesis. Values of scalar irradiance and chlorophyll a concentration were obtained directly from the moored sensors, values for the specific absorption coefficient of the phytoplankton were obtained by linear interpolation of shipboard measurements, and the quantum yield of photosynthesis was calculated as a function of scalar irradiance. Comparisons of photosynthetic rate calculated from such a formulation with measured rates of carbon assimilation indicated good agreement, with no apparent or easily explained biases in the predictions. Surprisingly, daily values in both the crop of phytoplankton and gross photosynthetic rate varied by only a factor of 2 during the year. The annual rate of gross photosynthetic production at the mooring was 12 mol C m−2. An analysis of errors suggests that the precision of this estimate is about ±20%.

The fluid mechanics of copepod feeding in a turbulent flow: A theoretical approach

Abstract An important area of biodynamics research is the interaction between predator and prey in nature. Several scales are significant for interactions between predator and prey over the life cycle of each organism. A key factor is the encounter probability (group and individual). On the basis of physical considerations, the group encounter probability depends upon the respective patch sizes (on the order of 10s of km) and their relative dispersion (or aggregation) rates in turbulent systems. The encounter probability at the individual level is affected by the relative motion of the predator and the prey and is controlled by the velocity spectrum. In addition, at the individual level, the striking distance of a predator will depend on motility and perception of the prey. Here we address the mechanics of copepod predation on phytoplankton and the coupling with the physics of turbulent fluid motions. Our aim is to review pertinent fluid dynamics, on scales of less than a few metres, to provide a framework in which to consider the role of fluctuating fluid velocities on copepod feeding.

Sediment transport and channel adjustments associated with dam removal: Field observations

[1] This study documents changes in channel geometry, bed level profile, and bed grain size distribution and their relations with the sediment transport at the reach scale, following the removal of a low-head dam. After the removal, net sediment deposition occurred downstream of the dam, and net erosion occurred in the reservoir, but approximately less than 1% of the sediment stored in the reservoir was transported downstream. No bank erosion was evident either upstream or downstream of the dam. Bed deposition and scouring in the reservoir accounted for a decrease in the bed slope of 30%. The stations downstream of the dam had surface bed material sizes at least 40% finer than preremoval conditions. However, the sediment transport rates downstream of the dam were not significantly different from predam to postdam removal or from an upstream control. Overall, the removal of the dam had only minor effects on the channel adjustment downstream of the dam. A simple analysis linking transport to channel geometry explains this effect.

The effect of temporal undersampling on primary production estimates

Annual primary production estimates for specific oceanic regions have typically been made using a variety of measures of productivity spaced, at best, several weeks apart Primary productivity in the oceans is known to be extremely episodic. It is hypothesized here that primary production data with a temporal resolution of several weeks have a high potential for error due to undersampling. In the present analysis, time series of gross primary productivity were calculated using time series of photosynthetically available radiation and chlorophyll a concentration as input to an optical production model. The input data are of minute scale resolution and were gathered during a number of moored experiments. These took place over the past 5 years at several oceanic sites. The minute scale productivity time series were integrated to form time series of daily estimates of gross production. These range in duration from 40 to 260 days. The time series exhibit several regimes characteristic of oceanic primary productivity, such as phytoplankton blooms, productivity pulses associated with advected water masses, steady state growth, and development of a subsurface productivity maximum. The presence of these features makes our time series ideal for investigating (1) the sensitivity of annual production estimates to the timing of the sample set and (2) the error introduced by undersampling inherent in coarser sampling methods. It was found that distinct pulses of productivity generate the greatest error and that high variability leads to large errors, even for well-resolved sampling intervals. The maximum percent error due to undersampling was found to be 85%. Additionally, up to a fourfold range between the maximum and minimum estimates of average daily production was found over all sampling intervals. Finally, the maximum expected range (300 g C m−2 yr1) and the expected standard deviation (±42 g C m−2 yr1) for annual water column production were determined at a Sargasso Sea site for which long-term productivity time series were available at four depths within the euphotic zone.

The role of advection and turbulent mixing in the vertical distribution of phytoplankton

The purpose of this study is to analyse the role of the flow field on the horizontal and vertical distributions of different phytoplankton populations thriving in the water column of a shallow coastal ecosystem. Two extreme flow conditions are illustrated. The first was a low energetic flow, under calm meteorological conditions and a stratified temperature of the water column. The second flow, coincident with the passage of a storm front, was more energetic resulting in increased mixing that homogenized the temperature in the whole water column. Although the mixing level homogenized the temperature of the water column in the high-energy period, it was not enough to homogenize the temperature in the low-energy period. In contrast, in both periods, the mixing level was enough to homogenize the vertical distribution of particles. A decrease in the concentration of particles from the calm period to the high-energy period was attributed to an advection event with warmer water of lower plankton concentration that resulted in a decrease of the total concentration of suspended particles in the water column. Data are used to test a model of plankton mixing proposed by Ruiz et al. (J. Plankton Res., 18 (1996) 1727).

Bio‐optical and physical variability in the subarctic North Atlantic Ocean during the spring of 1989

A unique set of physical, bio-optical, and meteorological observations were made from a mooring located in the open ocean south of Iceland (59°29.5′N, 20°49.8′W) from April 13 to June 12, 1989. The present measurements are apparently the first to resolve the rapid transition to springtime physical and biological conditions at such a high latitude site. Our data were collected with bio-optical and physical moored systems every few minutes. The abrupt onset of springtime stratification was observed with the mixed layer shoaling from ∼550 m to ∼50 m in ∼5 days. During this period a major phytoplankton bloom occurred with a tenfold increase in near-surface chlorophyll concentration in less than 3 weeks. Our statistical analysis indicates that the velocity shear in the upper layer is driven primarily by local wind stress. Mesoscale variability is also apparent from these and concurrent airborne oceanographic lidar observations. Our complementary modeling results suggest that the near-surface layer may be reasonably well described by a one-dimensional model and that the spring bloom was initiated during incipient near-surface restratification.

Trapped, near‐inertial waves and enhanced chlorophyll distributions

Waves with near-inertial frequencies were observed along a front associated with a large mesoscale feature in the Sargasso Sea during the late summer of 1987. High subsurface chlorophyll concentrations occurred on the edge of this front, coincident with the wave packets. The amplitude of the waves increased with time, and kinetic energy propagated downward, reducing 20-m Richardson numbers in the thermocline to 1 or less. Chlorophyll levels were episodic, showing no periodicity coincident with wave dynamics. However, on two occasions, chlorophyll concentration increased from 1 mg Chl m−3, several hours after the waves penetrated the thermocline. It was hypothesized that mixing associated with shear instabilities stimulated new production. A diffusivity model combined with nutrient data produced a phytoplankton bloom that accounted for only one of the maxima. The other increase in chlorophyll may have been the result of horizontal advection of the wave packets near the front.