Zooplankton Transport Research Articles

A mechanism for horizontal zooplankton transport by vertical migration in tidal currents

A mechanism is proposed by which net horizontal transport can be induced in an organism which migrates vertically in a purely oscillatory, vertically sheared tidal current. The mechanism contains two elements. First, for all reasonable tidal current profiles, net horizontal transport is induced in any organism which migrates vertically with a period which is an exact multiple of the tidal period. (This is the basis for the widelyknown process of selective tidal stream transport where the migration period and the tidal period are exactly equal.) The second element in the new mechanism is the observation that diel migration, the most common form of vertical migration, has a period of 24 h and is therefore an exact multiple of the principal solar semi-diurnal tidal constituent (S2) which has a period of 12 h. This relation between the S2 and diel migration periods stems from the fact that both phenomena are independently locked to the solar cycle. Diel vertical migration can therefore interact with the S2 tidal current constituent to induce longterm horizontal transport in migrating organisms. For reasonable tidal current and vertical migration parameters it is found that horizontal transport rates of 4 km d−1 are possible. The direction of net transport depends upon the phase of the S2 tidal current relative to local noon. The hypothesis is therefore proposed that geographical variation in S2 phase is a factor responsible for creating regions which are either retention-favorable or otherwise for diel-migrating marine organisms.

Genetic tracers of zooplankton transport in coastal filaments off northern California

Genetic markers were used to describe the transport of the calanoid copepod Metridia pacifica in a cool filament off Point Arena, California, in July 1986. The markers used were genetic variants of enzymes (allozymes); individual copepods were assayed for allozymic variability at nine enzyme‐encoding genes (loci) by polyacrylamide gel electrophoresis. Zooplankton samples were collected in a 300 km by 170 km domain. Maps of surface dynamic height in this domain showed an offshore jet, with strong cross‐shore transport, in the presence of a cool anomaly comprising multiple fronts and filaments. The genetic data sorted the samples into two heterogeneous groups (according to maximum likelihood analysis): one comprising samples near the coast and at the head of the filament, and the other comprising offshore samples. The groups themselves were heterogeneous, revealing smaller‐scale patterns of transport and mixing. The genetic spatial structure of the calanoid copepod, Metridia pacifica, collected in the flow field was used to infer patterns of transport in the filament flow field. Patterns of transport inferred from genetic differences between zooplankton samples were compared with expected particle transport based on models which consider physical (geostrophic advection, Ekman drift, and mixing) transport plus vertical migration behavior. According to both the flow models and the genetic data, transport of zooplankton occurred primarily along paths of geostrophic flow. There was little cross‐jet exchange and almost no exchange between samples in the filament and samples in a cyclonic eddy to the south of the jet. The genetic data further indicated that copepod populations at the coastal source of the filament were genetically heterogeneous. Advection in the filament thus resulted in the juxtaposition of genetically distinct zooplankton populations in the jet axis. The genetic structure of zooplankton in the filament resulted from advection of previous distant populations, not (because of the time scales of the processes) from changes in the genetic character of samples during transport. The presence of multiple fronts and filaments within the cool anomaly prevented homogenization of the plankton assemblages during transport. Thus samples collected less than 10 km apart across the jet axis were genetically different. These findings indicated that plankton populations in complex flow fields may show patchiness in biological, biochemical and/or genetic character at small time/space scales.

Zooplankton abundance in the lower Orinoco River, Venezuela

Zooplankton were sampled quantitatively in the lower Orinoco River and three major tributaries (Apure, Caura, Caroni). Dominant taxa were euplanktonic, which suggests origin from standing waters. In the Orinoco, densities for rotifers (mean, 7.6 ind. liter−1) were greater than those for cladocerans (0.9 ind. liter−1) or copepods (1.2 ind. liter−1), but cladocerans accounted for more than halfof total biomass (0.5 µg C liter −1). Average tributary contributions to zooplankton transport in the lower Orinoco (873 kg C d−1) were proportional to their average discharges. Extensive fringing floodplain along the lower Orinoco contributed very little to total transport. At low water in the Orinoco, when there was no contact with the floodplain, the transport of copepods (mainly nauplii) and Bosmina increased downstream by more than could be ascribed to additions from the tributaries or to reproduction in transit. These zooplankton appear to originate along the river margin, within the channel, and are probably significant as an energy input for food webs in the channel and as an inoculum to floodplain habitats at the time of inundation.

Zooplankton abundance and transport in a tropical white-water river

Zooplankton abundance and transport were studied in the Apure River of western Venuzuela over a 15-month period. Much of the lower portion of the drainage basin, which is an extensive savanna of low relief, is subject to seasonal flooding. Although more than 50 zooplankton species were recorded during the study, nine rotifer species accounted for more than 90% of total density (mean, 138 individuals · L-1). Copepods were represented primarily by Mesocyclops decipiens. Most cladocerans were planktonic, but cladocerans were not abundant. Crustaceans comprised 46% of annual mean zooplankton biomass (1.9 μgC·L-1) but only 2% of zooplankton numbers. The annual transport of zooplankton biomass from the watershed was 29 300 kgC. Zooplankton abundance showed a strong, inverse relationship to discharge. Secondary branches (canos) of the braided channel appeared to be the primary source areas, but populations of some species also reproduced in the main channel at low water. As the river began to rise, canos were flushed and thus abruptly ceased to serve as source areas. Zooplankton transport fell sharply and remained low until the river inundated the floodplain. Secondary production in the floodplain was exported to the river as long as a connection existed. After drainage ceased from the floodplain, transport fell to very low levels until canos again became suitable habitat. Seasonal fluctuations in river level regulate the development of source areas suitable for zooplankton growth and control the export of plankton from the source areas. Plankton in the running waters reproduce only at low water.

Distribution of zooplankton in a coastal upwelling in New Zealand

Abstract Species composition and biomass of plankton samples taken on transects across an upwelling plume off Farewell Spit in February 1981 are presented. Copepods were numerically dominant, particularly Oithona similis and Paracalanus indicus. Also abundant were large phytoplankters (Chaetoceros sp. and Trichodesmium sp.), crustacean faecal pellets, and euphausiid larvae. The distribution of zooplankton species suggests mixing of inshore waters and neritic populations with upwelled waters. These data are discussed with respect to an interpretation of the upwelling system as a northwards transport of zooplankton and its enhanced productivity into the South Taranaki Bight.

The transport and retention of zooplankton in relation to a sill in a tidal estuary

The transport and retention of zooplankton in relation to a sill in a tidal estuary

Recurring aggregation of Zooplankton by a tidal current

Zooplankton densities up to 40 times greater than those in adjacent wateraccumulated in the lee of a small point protruding into a strong tidal current at Whitsunday Island, Queensland, Australia. The density of plankton in the lee was linearly correlated with current velocity past the point, reaching maximum concentrations at mid-flood tide and decreasing as slack tide approached. Zooplankton did not accumulate when tidal flow reversed direction. Possible mechanisms of aggregation are discussed. Mass transport of zooplankton into lees by fine-scale current patterns may be a major cause of plankton aggregation in some near-shore localities and may significantly affect the distribution of fish and benthic invertebrates.