Loren R. Haury

Patterns and Processes in the Time-Space Scales of Plankton Distributions

It is evident that organisms have aggregated, patchy distributions of abundance on a wide variety of space and time scales. This can easily be seen in the terrestrial and littoral environments. It has been more difficult to observe in the pelagic realm simply because we cannot see into the ocean. Thus we must depend upon sampling to gain an impression of the space-time scales of pattern in this habitat. Despite the difficulties, enough sampling of the right sort has now been done so that we can make some very general statements about the nature of pattern in the ocean, particularly with regard to pattern in the distribution and abundance of planktonic organisms. All the evidence indicates that plankton is patchy on a broad spectrum of scales. Because this aggregated spatial pattern is such a general phenomenon, there is little question of its ecological and evolutionary importance. Further, because we assume our samples represent a larger universe, patchiness strongly affects our efforts to obtain estimates of the abundance of organisms and our ability to detect significant spatial and temporal changes in abundance. It is therefore of great importance that we understand its nature, causes, and effects.

Interactions of migrating zooplankton with shallow topography: predation by rockfishes and intensification of patchiness

Abstract Stratified net tows, multiple-frequency sonar and drifter drogues, combined with analysis of rockfish stomachs, were used to investigate the processes taking place when vertically migrating plankton are swept onto a shallow bank in the Southern California Bight. Euphausia pacifica, the dominant migrating plankter near the bank, was also a dominat item in the stomachs of Sebastes spp. when currents in predawn hours advected water from deep water toward the bank. In the morning, schools of 12 kHz scatterers, apparently rockfish, ascended tens of meters above the bottom toward the descending plankton. Net tow estimates of euphausiid biomass and the acoustic backscattering at 200 and 520 Hz were much lower above the bank than in a nearby deep-water area, particularly during early evening. At night and during predawn hours, a relatively high abundance of zooplankton, with densities similar to the deep-water area, was usually found near the edge of the bank. Causes of the observed recurrent lack of migrating plankton above a seamount in early evening could be diurnal predation by epibenthic fish or daytime advection of migrators around the seamount, thereby creating a “hole” above it. The formation of such “holes” can intensify the regional patchiness of plankton near shallow seamounts. Historical data of CalCOFI night-time tows show that the biomass of euphausiids, but not that of the weakly migrating chaetognaths, is significantly more variable (patchy) at stations near offshore shallow topography than at nearby deep-water stations. The similarity in the mean biomass of euphausiids at the two station sets suggests that the contribution of predation to the observed intensifications of patchiness may be insignificant.

Tidally generated high-frequency internal wave packets and their effects on plankton in Massachusetts Bay

Tidally generated internal wave packets occur twice a day during late summer in Massachusetts Bay, U.S.A. The packets are formed at Stellwagen Bank and propagate into the Bay at about 60 cm sec-; they dissipate in shallow water at the western side of the Bay. The dominant waves in packets have lengths of about 300 m, periods of between 8 and 10 min, and amplitudes of up to 30 m. Overturning of the waves has been observed acoustically over Stellwagen Bank, in the deep (80 m) waters in the center of the Bay, and during dissipation in shallow water. The effects of the internal waves on the distribution of plankton were investigated in August 1977 using an instrument package (Longhurst-Hardy Plankton Recorder, in situ fluorometer, CTD) towed either at a constant depth or following an isotherm through wave packets. Phytoplankton and zooplankton appear to be carried passively up and down by the internal waves; the data were insufficient to resolve any active response by zooplankton to vertical displacements by the waves. Vertical distributions of the plankton were altered by overturning of waves and subsequent mixing. Patterns of horizontal distributions of plankton determined by constant-depth tows were dominated by the effects of internal wave vertical displacements. Isotherm-following tows removed much of the variability due to wave displacement, but problems of following rapidly moving isotherms introduced considerable smallerscale variability. Changes in zooplankton abundance on tow length scales (600-1200 m) were not correlated with temperature, salinity, or density; some significant correlations with chlorophyll fluorescence occurred when internal wave activity was present. Twice a day for several hours or more, phytoplankton were vertically displaced by as much as 30 m, with a period of about 10 min. The light level plant cells experienced varied from 0.1 to 26% of the ambient surface illumination. This rapid change in light should alter fluorescence yields of plant cells and affect continuous in situ measurements of chlorophyll fluorescence. The timing of internal wave packets varies with the semidiurnal tidal cycle and therefore interacts with the day-night cycle to significantly alter the light regime experienced by plant cells over a two-week period. This could affect the primary productivity of the Bay in the area affected by internal wave packets. The sporadic occurrence of internal wave overturning and mixing events in the deep waters of the Bay could enhance primary production by increasing nutrient input to the mixed layer. This effect should be greatly enhanced in the shallow waters where the internal waves 1. Scripps Institution of Oceanography, La Jolla, California, 92093, U.S.A. 2. Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, 02543, U.S.A. 3. Gulf Research and Development Company, P.O. Box 2038, Pittsburgh, Pennsylvania, 15230, U.S.A.

Zooplankton patch dynamics: daily gap formation over abrupt topography

Abstract Net tow and acoustic surveys of zooplankton distributions were made over and around Sixtymile Bank (110 km southwest of San Diego, California). Gaps devoid of vertically migrating zooplankton were formed every evening above the summit of the bank. Interactions between the migrating animals, their predators, physical advection and the local topography appear to determine the gap formation and dynamics. Gaps were transported downstream during the night and appeared to disintegrate slowly through vertical swimming behavior, current shear and mixing processes. Patch dynamics following gap formation, mediated by both ocean currents and animal behavior, should augment the spatial heterogeneity of zooplankton and affect marine food webs in areas where abrupt topography features are common.

Effects of turbulent shear flow on zooplankton distribution

Abstract Dynamics of the physical environment should help regulate interactions within zooplankton communities that determine their structure and function. Using a submarine, we have made the first simultaneous measurements of turbulent dissipation rate, vertical velocity shear, temperature, salinity, and zooplankton distribution and abundance. Our results suggest that energetic turbulent regimes due to strong winds, combined with an anomalous region of high vertical shear, broaden the vertical distributions of some zooplankton species, resulting in the selective mixing together of species that are vertically separated under less energetic conditions.

An offshore eddy in the California current system Part IV: Plankton distributions

Abstract The distribution of plankton across a warm-core eddy system in the California Current 400 km off Point Conception, California was studied in January 1981. The eddy system, about 150 km in diameter at the 7°C isotherm, was made up of a 75 m thick surface layer, a cold-core region extending from 75 m to about 200 m, and a warm-core eddy below 200 m extending to at least 1450 m. Casts for the vertical distribution of chlorophyll/phaeophytin and integrating zooplankton net tows were taken at 37 stations located about 20 km apart on two orthogonal transects across the eddy system. Vertical distributions of microplankton were determined on one section from the eddy center to beyond the eastern edge. Integrated chlorophyll/phaeophytin values were highest to the north and east of the eddy system; across the system itself, there was only a small increase of values near the center. Asymmetrical distributions of maximum concen Current water was being entrained into the center of the eddy system from the northeast. Dinoflagellates were numerically the most important member of the microplankton, especially in the deep chlorophyll maximum. Zooplankton distributions indicated the intermingling of warm and cool water species throughout at least the upper 200 m of the eddy system. Some cold water species were as abundant inside the system as outside to the north and east; their numbers were much reduced in a band surrounding the system where warm water species were most abundant. The presence of species characteristic of different water types throughout the region of the eddy system provides an indication of the mixing that had occurred since the system originally formed. The biological data, together with the physical and chemical results, indicate the importance of frontal boundary processes and lateral entrainment of surrounding water into the eddy system in determining the character and productivity of such systems.

Zooplankton distribution around four eastern North Pacific seamounts

Abstract The effects of seamounts on the distribution of zooplankton were investigated at four seamounts in the northeastern Pacific. The following hypotheses were tested: (1) mesoscale gaps of reduced abundance of migrating zooplankton develop over seamounts every night; (2) fine-scale patchiness is augmented within these gaps and in the region downstream of seamounts; and (3) increased numbers of crustacean carcasses occur over seamounts. Gaps are expected because most zooplankton that descend over shallow topography at dawn are either eaten by resident predators or are advected off the summit, while fine-scale patchiness should result from lateral shear between the gap and the surrounding zooplankton-rich waters. Copepod carcasses should be more abundant over seamounts than the surrounding water because of the increased predation at seamounts. Zooplankton distributions were determined from net samples and acoustic records. Water column properties were measured with a CTD and hydrocasts, and currents by moored current meters, acoustic current profilers and drifter drogues. Zooplankton gaps were found over three of the four seamounts surveyed, but not on every survey of each seamount. Only three of the surveys provided the information necessary to test the patchiness hypothesis; on two of these increased patchiness and carcass abundance were found associated with gaps. When no gap was observed over a seamount, there was no evidence of increased carcass abundance or enhanced patchiness, indicating that the three phenomena are temporally and spatially linked. Copepod carcasses were found in the intestines of small fish sampled at the same time as the zooplankton. The fish, along with crustacean predators like euphausiids, are the likely source for the carcasses. Seamounts appear to be an important cause of enhanced zooplankton patchiness on scales ranging from 100s of meters to areas larger than the seamounts themselves.

Fine-scale multi-species aggregations of oceanic zooplankton

Abstract Sixteen Longhurst-Hardy Plankton Recorder tows were taken at different depths in the northwest Atlantic for analysis of fine-scale horizontal patchiness. Abundant species were non-randomly distributed in patches with scales of tens to hundreds of meters. Positive correlations between species abundances dominated, indicating that the patches were multi-species associations. Most horizontal pattern appeared to be of biological origin.

Synoptic three-dimensional circulation in an onshore-flowing filament of the California Current

Abstract Hydrographic (CTD and XBT casts) and Lagrangian particle drift (satellite-tracked mixed layer drifters) measurements taken off Southern California and northern Baja California in July 1985 are in the region where, historically, a stable, broad onshore flow is indicated in the 0/500 db relative dynamic topography. A 50 km wide onshore-flowing filament of cold and low-salinity water is resolved by the new measurements. Water mass properties (temperature, salinity and spiciness) are used to trace the different water masses and describe the circulation in the surveyed region. A comparison of the 15/300 db relative geostrophic currents to the drifter-inferred velocities shows a significant absolute geostrophic flow at the 300 db level, with coherent patterns similar to the surface mesoscale circulation. Estimates of vorticity calculated from the geostrophic and Lagrangian data are in qualitative agreement. Surface divergence and the implied vertical velocities of the order of 0.5 m day −1 are resolved at scales of 50–100 km. The spatial distribution of these estimates shows significant upwelling within the core of the filament.

Longhurst-hardy Plankton recorders: their design and use to minimize bias☆

Abstract Field studies and tow basin experiments of Longhurst-Hardy Plankton Recorder (LHPR) biases were conducted by injecting various types of particles into an LHPR having a variety of net and recorder box configurations. Tow speed and clogging were also considered. The following factors contributed to biases: (1) low recorder gauze aperture area to throat area ratios, causing backing up of flow into the net cod end, (2) tunnel gauze slot design, allowing significant losses of particles; (3) steep side angles of the net at the recorder entrance due to bagging or radial stretching; (4) clogging of the neta head of the LHPR; (5) short nets; and (6) tow speeds greater than about 125 cm s−1. Experiments with an LHPR modified to minimize biases showed that resolution to 15 m in oligotrophic waters is possible. In eutrophic waters, we doubt it is feasible to use a net with the LHPR.