Open Coastal Waters Research Articles

Small-scale zooplankton community variability in a northern British Columbia fjord system

We have used multivariate classification, ordination, and discriminant function analyses to describe the small-scale spatial pattern of zooplankton community composition in a northern British Columbia fjord system. Finely ground tailings from a molybdenum mine were discharged into one branch of the fjord (Alice Arm) during a 17-month span which included our two sampling periods (August 1981 and June 1982). Our study had two major objectives. The first was to learn whether or not the zooplankton community was strongly altered in the vicinity of the tailings discharge, and the second was to characterize the intensity and dominant spatial grain scale of ‘normal’ community variability in the remainder of the deep inlet system. Zooplankton were collected by water column integrating oblique net hauls. The statistical analyses examined relative and absolute changes in the contribution of individual species to the local zooplankton community. Within each time period, the range of spatial variability was small in comparison both to the between-time period (probably seasonal) differences within the inlet, and to the range of spatial variability observed by identical analysis methods in open continental shelf waters to the south. However, despite this small overall community variance, we found a reasonably consistent regional partitioning of the inlet; the samples most similar in composition tended to be those from the same or adjoining locations. Sites near the turbid heads of the branching inlet (including those near the tailings discharge) had higher concentrations of Euphausia pacifica and other large non-copepod zooplankton, and relatively low concentrations of the copepod Neocalanus plumchrus. The magnitude of the between region-differences was small and did not appear to be increasing with time; we therefore conclude that the impact of tailings discharge on zooplankton community structure was relatively minor. The distance over which neighbouring samples show significant compositional correlation is smaller in the inlet than in open coastal waters previously studied; we interpret this as due largely to the absence of meso-scale horizontal turbulent eddies within the inlet.

The fauna associated with drift algae captured with a plankton‐mesh purse seine net1

A plankton‐mesh purse seine used to encircle drift algae in open coastal waters was compared with seines in open water and conventional ichthyoplankton hauls. Densities of small fish and invertebrates were higher in association with drift algae than in open water. Most fish that occurred with drift algae were well pigmented. The fauna of drift algae differed from that of attached algae. Large numbers of epiphytic animals vacated attached algae which were experimentally detached from the substratum. The fauna on drift algae, therefore, is largely the result of colonization from open water. Fish and invertebrates were quickly attracted to clumps of experimental algae that were left to drift. This suggested that drifting algae may quickly influence the distribution of animals in open water.Appreciable quantities of drift algae may influence the survivorship and recruitment processes of species which associate with algae during the planktonic phase of their life cycles.

Wind-driven circulation of Cockburn Sound

The water circulation within the Cockburn Sound embayment in Western Australia is predominantly wind-driven. Observations by moored current meters showed the mean velocities to be less than 0.05 m s-1 within Cockburn Sound and 0.07 m s-1 in the adjacent open coastal waters. Maximum tidal currents were very low, with an amplitude of the order of 0.01 m s-1. The importance of the wind-forcing was evident in the time-history data collected by the wind and current meters, particularly under storm conditions when winds greater than 15 m s-1 produce currents, within the Sound, between 0.10 and 0.25 m s-1. The wind records from Fremantle were examined and recurring patterns, such as sea breeze on and winter storms, were identified and their annual frequency of occurrence was estimated. A time- dependent, vertically integrated wind-driven numerical model was used to simulate the various water circulation patterns of each wind category. The circulation Froude number was shown to be of the order of thus allowing a description in terms of the stream function. Correlation between the data collected by the moored current meters and predicted by the model ranged between 0.05 and 0.76. Profiling data showed that under near calm wind conditions (<2 m s-1), local horizontal density gradients, caused by evaporation and heating, produced currents of up to 0.1 m s-1. This inverse estuary nature of the flow appears to play a significant role in the dynamics at low wind speeds. Under these conditions there was little or no volume exchange with the open sea. By contrast, the wind-driven circulation model showed that under a sea-breeze pattern 1.4 × 107 m3 was typically exchanged diurnally, and a 2-day winter storm may exchange about 1.1 × 108 m3, which is small compared with the volume of the Soand (1.2 × 109 m3). The bathymetry is such that the Sound acts mainly as a closed system.

Recreational-Boating Patterns and Water-Surface Zoning

IN several locales the coastal waters of the United States are heavily trafficked by both recreational and commercial vessels. This dual usage carries the possibility that competition will become conflict and occasionally disaster. One method to mitigate conflicting use is water-surface zoning that seeks to restrict or ban certain types of vessels at specified times or places. Optimal implementation of water-surface zoning requires a knowledge of how and where a water surface is used, particularly by boaters, who constitute the largest single group of users. Because of this need for data on use, the application of watersurface zoning has been confined to small water bodies where vessels can be inventoried and their patterns of activity charted without too much difficulty or expense. In contrast, zoning is sparingly applied to open coastal waters partially because it is more difficult to acquire adequate data about the use of their surfaces.

The effects of predation by wading birds (Ardeidae) and blue crabs ( Callinectes sapidus) on the population size structure of the common mummichog, Fundulus heteroclitus

Preliminary observations suggested that predation by herons and egretsaffected the size structure of a mummichog (Fundulus heteroclitus) population in Tar Landing Marsh, North Carolina. The hypothesis was tested by placing 300 mummichogs represented equally in three size classes into each half of an enclosed, divided, high marsh pool. Nylon cord was strung in a grid over one side to inhibit predation by wading birds. Although mummichog losses were higher in the side of the pool which was open to bird predation, the same trend in size-specific losses also occurred in the bird-exclusion side. Repeating the experiment after the removal of several blue crabs (Callinectes sapidus) improved mummichog survivorship and suggested that crabs and not birds were responsible for the fish losses previously observed. In a final experiment, birds were excluded and crabs were removed from one side but added to the other side of the pool. After 24 days losses due to C. sapidus occurred in all mummichog size classes, but size-specific predation by blue crabs produced the highest losses (90%) among the largest (>70 mm total length) fish. Blue crab predation on mummichog populations may influence the community structure of salt marsh infauna and may also function in the movement of marsh production to open coastal waters.

伊勢湾とその隣接水域におけるラスバンマメガニ幼生の分布

The objects of this study are to clarify the temporal and areal distribution of larvae of Pinnixa rathbuni in Ise Bay and its neighbouring coastal waters, and to determine to what extent the final ecdysis into megalops takes place in water without drifting into the oceanic environment. The larvae were found in November to March of the following year. Early zoeal larvae appeazed at first abundantly in November, and then abundant megalops were found in February of the next year, with the result that the length of the larval period in the plankton was approximately 120 days. This period is from two to four times that required for the larval development of the pinnotherid crabs synbiotic with invertebrates. The larvae were found abundantly in Ise Bay and Mikawa Bay, but they were not met with in the open coastal and oceanic waters except the area near the mouth of the bays. This suggests that the larval dispersal into the open coastal and oceanic region does not considerably influence the loss of the larval population from the bays.

Zooplankton Distribution in Relation to the Hydrology of the Derwent River Estuary

The zooplankton of the Derwent River estuary was sampled during periods of high freshwater flow (October) and low freshwater flow (April). Zooplankton distribution was found to be similar at both times with only minor shifts in location. Observed plankton distribution is shown to correlate well with a hydrologically established zonation scheme. Despite continual exchange between oceanic, coastal and inshore coastal waters, there is only occasional estuarine penetration into the inshore region and little penetration of inshore plankton into the estuary. The tidal zone is shown to be stable, well defined and characterized by estuarine and marine species. It is suggested that in a similar estuarine system, where the zooplankton distribution is well known, the hydrological zones can be inferred. The estuary of the Werribee River, Victoria, contains most of the same copepod species and is similar in other ways to the Derwent River estuary. But the occurrence of indicator species in the marine region of the Derwent estuary suggests that it is subject to a stronger influence of open coastal and oceanic waters.

Changes in the ecological balance of invertebrate communities in kelp holdfast habitats of some polluted North Sea waters

1. This paper presents a thirty-one month study of a neotenous invertebrate community which developed inLaminaria hyperborea holdfasts in some polluted waters of the North Sea (northeast England und southwest Scotland). 2. Sixty miles of open coastal waters and approximately thirty miles of the Forth Estuary are affected by pollutants. 3. Reduction in the species diversity leaves only one possible detritus food chain in the habitat. 4. This simple community is not in balance. 5. The ecology of the polluted water community is considered and the possible long-term effects of pollution in the kelp forest are discussed.

Impact of the extent and locality of mud patches on the density and geograpidc distribution of juvenile Agulhas soleAustroglossus pectoralis(Soleidae)

The main focus of this study was to gain an understanding of the distribution of juvenile (10–24 cm Lt ) Agulhas sole and some factors that could govern their dispersal and migration. The hypothesis that the size and the locality of mud patches determine the density and geographic distribution of juvenile sole was investigated. Data from 10 trawling surveys, between 20 and 27°E along the southern Cape coast during the period 1986–1992, were analysed to assess the density and geographic distribution of juvenile Agulhas sole. Within the distribution area of Agulhas sole there are two localities which differ widely in the size of mud patches. Agulhas sole do not appear to be dependent on estuaries as nursery grounds, but juveniles were more abundant on the extensive mud patches of open marine coastal waters of the central Agulhas Bank than in large sheltered bays, with limited patches of mud, farther east. Spawning adults were found mainly on sandy sediment close to mud patches or farther offshore. The locality of spawning, in relation to the current dynamics, suggests that, on the inner shelf of the central Agulhas Bank, spawning products would not be dispersed by the Agulhas Current. However, towards the east, spawning products could be advected eastwards and sometimes be entrained in the eddies of the Agulhas Current.

Movements of Brook Trout, Salvelinus fontinalis (Mitchill), Between and Within Fresh and Salt Water

Major movements of trout into Ellerslie Brook, Prince Edward Island, were in April, June–July, and November; and out in May and October to early January. Some movements were preponderantly in one direction; others involved trout making simultaneous movements up- and downstream. Mean length of trout in movements was 16.8 cm., predominantly age II. Trout were short-lived and few survived to make repeated movements. About 50% of trout making return movements did so within a month. Percentage of Ellerslie-reared trout which ran to salt water varied from 12 to 35 over a 6-year period. There was inconclusive evidence of a heritable propensity to sea-running. After descending into the estuary, few trout left for open coastal waters. Short-distance movements resulted in population shifts in the brook in summer which were not detected in traps at mouth of stream.Movements of trout between fresh and salt water were very closely associated with rise and fall of the water level in the brook, but not with height of water. Movements in spring and early summer were into preferred temperature. Although marked changes in water level occurred, few trout moved in winter after becoming acclimated to a water temperature of 0 °C. or in summer when brook was at temperature of final preferenda. Continuance of movement for some time after 0 °C. was reached seemed related to slow rate of acclimation. Movements were both random and directed. For most movements trout appeared conditioned by temperature and stimulated by changing water levels. Trout moved largely at night. Other influencing factors were living space, maturation and spawning. Turbidity, salinity, tides, and handling were without apparent effects.