Showing papers on "Pelagic zone published in 1991"

Effect of Temperature and Size on Development, Mortality, and Survival Rates of the Pelagic Early Life History Stages of Marine Fish

Abstract: This paper presents a comprehensive summary of development, mortality, and survival rates of fish eggs, yolk-sac larvae, and postlavae in relation to temperature and size. The general temperature- ...

The effect of temperature and algal biomass on bacterial production and specific growth rate in freshwater and marine habitats

Abstract: We analyzed heterotrophic, pelagic bacterial production and specific growth rate data from 57 studies conducted in fresh, marine and estuarine/coastal waters. Strong positive relationships were identified between 1) bacterial production and bacterial abundance and 2) bacterial production and algal biomass. The relationship between bacterial production and bacterial abundance was improved by also considering water temperature. The analysis of covariance model revealed consistent differences between fresh, marine and estuarine/coastal waters, with production consistently high in estuarine/coastal environments. The log-linear regression coefficient of abundance was not significantly different from 1.00, and this linear relationship permitted the use of specific growth rate (SGR in day−1) as a dependent variable. A strong relationship was identified between specific growth rate and temperature. This relationship differed slightly across the three habitats. A substantial portion of the residual variation from this relationship was accounted for by algal biomass, including the difference between marine and estuarine/coastal habitats. A small but significant difference between the fresh- and saltwater habitats remained. No significant difference between the chlorophyll effect in different habitats was identified. The model of SGR against temperature and chlorophyll was much weaker for freshwater than for marine environments. For a small subset of the data set, mean cell volume accounted for some of the residual variation in SGR. Pronounced seasonality, fluctuations in nutrient quality, and variation of the grazing environment may contribute to the unexplained variation in specific growth.

Reproduction of Antarctic benthic marine invertebrates: Tempos, modes, and timing

Abstract: Work on the life histories of common antarctic benthic marine invertebrates over the past several decades demands a revision of several widely held paradigms. First, contrary to expectations derived from work on temperate species, there is little or no evidence for temperature adaptation with respect to reproduction (gametogenesis), development, and growth. It remains to be determined whether the slow rates of these processes reflect some inherent inability to adapt to low temperatures, or are a response to features of the antarctic marine environment not directly related to low temperature, such as low food resources. Secondly, contrary to the widely accepted opinion designated as “Thorson's rule,” pelagic development is common in many groups of shallow-water marine invertebrates. In fact in some groups, such as asteroids,pelagic development is as prevalent in McMurdo Sound, the southern-most open-water marine environment in the world, as in central California. In other taxonomic groups, especially gastropods, there does seem to be a genuine trend toward non-pelagic development from tropical to antarctic latitudes. Although this trend has been predicted by theoretical models, its underlying causes appear to be group specific rather than general. Thirdly, pelagic lecithotrophic development, often considered to be of negligible importance, occurs in many shallow-water antarctic marine macroinvertebrates. Pelagic lecithotrophy may be an adaptation to a combination of poor food conditions in antarctic waters most of the year and slow rates of development. Nevertheless, some of the most abundant and widespread antarctic marine invertebrates have pelagic planktotrophic larvae that take very long times to complete development to metamorphosis. These species areparticularly prevalent in productive regions of shallow water (<30 m), which are frequently disturbed by anchor ice formation, and the production of numerous pelagic planktotrophic larvaemay represent a strategy for colonization. Although planktotrophic larvae tend to be seasonal in occurrence, their production is not linked particularly closely to the mid-summer pulse of phytoplankton production. These larvae show no evidence of starvation, even during times when phytoplankton abundance is very low, and they may depend on unusual sources of food, such as bacteria. How they escape the selective conditions that apparently led to a predominance of non-feeding modes of development in antarctic marine invertebrates remains as a major challenge for antarctic marine biology.

Physical Heterogeneity and the Organization of Marine Communities

Abstract: Benthic and pelagic marine habitats, like terrestrial landscapes, can be viewed as mosaics of environmental quality produced by spatial and temporal variation in the physical and biological constraints encountered by populations. In marine systems hydrodynamic processes (water column stability, temperature and nutrient gradients, turbulent oceanographic features, storm disturbances) and biological processes (competition, grazing, predation) affect the recruitment and survival of populations. The emerging view of the dynamics of communities and populations as nonequilibrium systems (Chesson and Case, 1986; DeAngelis and Waterhouse, 1987) increases the interest in processes that generate or perpetuate heterogeneity. That variable environmental parameters cause gradients in community structure or the distribution of species is by no means a novel observation. Merriam’s (1898) concept of life zones relating plant distributions to changes in air temperature along an elevation gradient was an early explanation of environmentally controlled community patterns. Terrestrial biomes (Clements and Shelford, 1939) are defined by large-scale changes in physical and biological characteristics of the landscape. In some marine habitats, however, these divisions may be less apparent, but they are nevertheless similarly heterogeneous over many scales in space and time.

Fish in Reservoirs

Abstract: In all reservoirs the fish fauna is recruited from their riverine antecedents. The nature of the riverine environment, created by its unstable hydrological regime, favours the evolution of generalist species. Of these species, only a few are preadapted to lacustrine conditions. When riverine fish communities are trapped in a reservoir, most of the fish stay close to the shore, the mouth of tributaries and in shallows. The pelagic and deep waters are poorly utilized and the yield of fish is below the natural productivity of these water bodies, especially in the case of deep and large reservoirs. This occurs unless the dammed river contains fish species from families which are preadapted for lacustrine conditions. A positive correlation between the percentage of pelagic fish and the total yield of fish in reservoirs indicates that the pelagial represents a vacant habitat, only seldom invaded by riverine species. This situation probably applies to the deep water of reservoirs too, although sufficient data to demonstrate this are not available. The littoral region of reservoirs is utilized by some generalists of euryoecious species derived from the indigenous riverine fauna which may be preadapted for lacustrine conditions. If such preadapted lacustrine species are not present, the introduction of lacustrine species or those preadapted for the lacustrine environment is desirable under certain circumstances. However, caution must be exercised in any introduction.

Trophic interactions between copepods and microplankton: A question about the role of diatoms

Abstract: Relationships among microplankton composition, copepod diet, and egg production are examined with data from gut content analysis of copepods from California coastal waters and from the Irish Sea, feeding and egg production experiments on Acartia tonsa off southern California, and egg production measurements on copepods from a subtropical estuary (Port Everglades, Florida), temperate shelf waters (southern California, Irish Sea), and the open ocean (Gulf Stream). The copepod species studied appeared to feed preferentially on dinoflagellates and microzooplankton relative to diatoms. Patterns of variability in egg production conform, generally, to changes in dinoflagellate and microzooplankton biomass, but seem to be independent of changes in diatom biomass.

Oceanic pelagic prey of benthopelagic fish in the benthic boundary layer of a marginal oceanic region

Abstract: The availability of oceanic plankton and micronekton to the benthopelagic fish assemblages on the slopes of the Rockall Trough, a marginal region of the northeast Atlantic deep-sea environment, is examined The vertical distribution of pelagic species predated by the fish was determined in the off-slope water column, as were the depths at which they are predated by the fish in the benthic boundary layer of the slope The benthopelagic fish predated pelagic species a t depths on thta slope corresponding to the daytime depths of the pelagic prey The occurrence of these pelagic species a t the benthic boundary layer is primarily through truncation of their pelagic vertical distributions rather than horizontal impingement, although this does occur: thls applies not only to epiand mesopelagic but also to the bathypelagic species which can even reach abyssal sediments Die1 vertical migration of the pelagic species did not make them available to slope fishes in shallower depths presumably because vertical migration is strictly upwards within the pelagic water column and not u p the contours of the slope sediment

Acoustic Measures of the Abundance and Size of Pelagic Planktivores in Lake Michigan

Abstract: Based on acoustic data taken at night and vertically stratified by bottom depth (3–110 m only), the total number (± 95% CI) of pelagic fishes in Lake Michigan was 43.4 ± 10.1 × 109 or 226.0 ± 55.2 kt in spring (mean density 0.7–3.8 fish∙m−2 or 1.6–12.8 g∙m−2) and 115.8 ± 18.3 × 109 or 313.2 ± 74.3 kt in late summer, 1987 (mean density 1.1–7.9 fish∙m−2 or 3.0–13.2 g∙m−2); approximately 30% of this increase in numbers (35% of biomass) occurred within Green Bay. Abundance estimates from horizontally stratified (by water column depth) data were within 9–11% of vertically stratified estimates during spring but over 20% higher during summer. By extrapolation to all water depths, we estimated total pelagic biomass as 274.6 kt for spring and 410.8 kt for summer. During both seasons, smaller fishes were nearer to the surface and nearer shore than larger individuals, and acoustic measures of size approximated the sizes of fishes caught in trawls. Bioenergetic model simulations suggest that 60% of the available prod...

Link Between Long-Term Variability in Upwelling and Fish Production in the Northeast Pacific Ocean

Abstract: The biomass of pelagic fish in the Coastal Upwelling Domain off the west coast of North America decreased by a factor of 5 in the first half of this century. We assemble several physical and biolog...

From Individual Plankton Cells To Pelagic Marine Ecosystems And To Global Biogeochemical Cycles

Abstract: It is often assumed that the biological CO2 pump in the oceans is essentially driven by the upward flux of NO3, so that the potential export of biogenic carbon from the upper ocean is stoichiometrically equivalent to N-derived phytoplankton new production. The steady-state/stoichiometry model does not consider that ecosystem dynamics have a significant influence on the export or sequestration of biogenic carbon. Contrary to this assumption, there are several cases where ecosystems do have a significant effect on the export and sequestration of biogenic carbon. These include the fixation of nitrogen gas in the upper layer of the ocean (phytoplankton and coral reefs), the export of carbon by organisms with high carbon content (thecate dinoflagellates, coccolithophores, foraminifers, pteropods), the production of long-lived dissolved organic matter, and the fact that the pathways of export may differ in length and complexity. This is considered within the context of a general typology of pelagic marine ecosystems, based on the various possible combinations linking standing stock to production of phytoplankton: (1) production and standing stock dominated by large cells (e.g. upwelling, ice-edge and episodic blooms); (2) production by small and large cells, standing stock dominated by large cells (e.g. exceptional blooms); (3) production and standing stock of small and large cells (e.g. spring bloom in the North Atlantic); (4) production by small and large cells, standing stock dominated by small cells (e.g. Alaskan Gyre in the North Pacific); (5) production and standing stock dominated by small cells (e.g. oligotrophic ocean). These five types of ecosystems correspond to different modes of phytoplankton production (as controlled by hydrodynamics) and different structures (as reflected in the standing stocks), which influence the pathways of carbon export and sequestration as well as the renewable marine resources.

Under-ice feeding and diel migration by the planktonic copepods calanus glacialis and Pseudocalanus minutus in relation to the ice algal production cycle in southeastern Hudson bay, Canada

Abstract: The marine planktonic copepodsCalanus glacialis Jaschnov andPseudocalanus minutus (Kroyer) typically dominate the copepod biomass in spring under the ice in southeastern Hudson Bay, Canada Females of both species exhibited significant diel feeding cycles, as measured by gut pigment content, throughout a bloom of ice algae at the ice-water interface in 1986 Periods of grazing correlated well with a nighttime vertical migration by females to within 02 m of the ice-water interface, suggesting that feeding took place at or just below the thermohaline boundary between seawater and the interfacial layer containing the ice algae Seasonal melting of the ice bottom in mid-May resulted in freshening of the surface layer and release of the ice algae into the water column FemaleC glacialis andP minutus responded by ceasing migration to the interface Gut pigment content, and by reasonable assumption, feeding activity in the water column, increased substantially immediately after this event In mid-May, the water column phytoplankton consisted of flagellates, sedimenting ice algal cells, and diatoms (Navicula pelagica andChaetoceros sp) previously found at the interface and then growing in the water column We conclude that algae growing at the ice-water interface, and sedimenting or actively growing algae derived from this interfacial layer, are a regular and principal source of nutrition for these pelagic copepods during and immediately after the ice algal bloom

Research on Antarctic Coastal Ecosystem Rates (RACER): an interdisciplinary field experiment

Abstract: The Research on Antarctic Coastal Ecosystem Rates (RACER) program of 1986-87 was designed to examine physical and biological processes that give rise to high biological productivity in the vicinity of the Antarctic Peninsula. In a 25,000 km2 sampling area at the western end of Bransfield Strait and during the period from mid-December 1986 to late March 1987, four 2-week cruises were made over a 69-station grid encompassing the coastal shelf, the continental shelf break, numerous island shelves, the Bellingshausen Sea-Weddell Sea confluence, and a portion of Drake Passage. Studies were focused on the upper ocean (0–200 m) and included detailed measurements of hydrography, ocean optics, vertical flux, and the distribution, abundance and productivity of bacteria, phytoplankton, zooplankton and krill. This paper describes the program logistics and presents the results of a carbon flux model, based on empirical observations, for the nearshore pelagic marine ecosystem.

Patterns of production and sedimentation in the boreal and polar Northeast Atlantic

Abstract: Pelagic systems are potentially capable of retaining and recycling all autochthonous organic material, although some losses due to sinking particles inevitably occur. Relating processes in the surface layers quantitatively to vertical particle flux is difficult because only a small percentage of the total production is lost annually via sinking in the open ocean. Further, only a few types of particles contribute to this flux and only a smalt proportion of these may actually reach greater depths. Measurements of vertical flux with sediment traps revealed seasonal and regional patterns also within the northwestern Atlantic and indicate imbalances between particle formation and degradation. The classical pattern of spring bloom sedimentation followed by reduced loss rates has been found in shelf and shallow water regions such as the Norwegian Coastal Current and fjords and is also encountered in the Barents Sea. In the Norwegian Sea. however, the seasonal pattern appears different as the seasonal maximum has been observed during late summer/autumn. The physical environment determines nutrient availability and hence the particles potentially available for sedimentation. The relationship between phyto- and zooplankton governs vertical flux seasonality, and zooplankters with different life cycles and feeding strategies further modify the principle patterns. Herbivores with life-cycle strategies involving overwintering of large biomass and predictable seasonal appearance (copepods, cuphausiids) will have a different impact than opportunistic organisms with very low overwintering biomass, for example salps and ptcropods. The latter exhibit much greater interannual biomass variation and may thus contribute to interannual variability of the vertical flux. Shelf systems of similar latitude arc generally comparable with respect to their flux patterns and also share similarities with marginal ice zones. Open ocean systems as the Norwegian Sea, however, exhibit different patterns which are similar to the subarctic Pacific.

Occurrence of the Toxic Phytoflagellate Prymnesium parvum and Associated Fish Mortality in a Norwegian Fjord System

Abstract: At the end of July 1989, toxin-producing Prymnesium parvum was spread through a fjord system in the surge of freshwater released from a hydroelectric power plant. In total, 750 tonnes of Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss) died in fish farms. Prymnesium parvum germinated in the brackish surface layer of a fjord branch which, during July, was characterized by longer residence time, higher temperatures, and lower nitrogen and silicate concentrations than the rest of the fjord system. Nutrient loading (especially phosphate) from fish farms may, however, have stimulated local growth of the alga. At the time of the first observed fish mortality, the salinity was 5‰ and the temperature 18 °C. Pelagic concentrations of P. parvum were generally low, with a maximum of 2.2 × 106 cells∙L−1 found close to a fish farm. Denser concentrations of P. parvum were, however, found in association with benthic substrates. Phosphorus limitation was probably important for the production of toxin...

Biomass size spectrum of the Lake Michigan pelagic food web

Abstract: Biomass size spectra for the complete Lake Michigan pelagic food web from picoplankton to salmonids were constructed for nine sampling transects around the lake in May and in September 1987. Size spectra were typical for freshwaters, having distinct peaks corresponding to major size groups. Biomass concentration of algae, zooplankton, and planktivores conformed to particle-size model predictions, but piscivore biomass was lower than predicted because these species are stocked. Mean annual total pelagic biomass was 72.3 g∙m−2 compared with a predicted range of 78.8–85.3 g∙m−2. Potential production of piscivores, Mysis, and Pontoporeia was in agreement with model predictions. No estimates of zooplankton or planktivore production were available, but we calculated that these could be 72.1–91.6 and 2.5–4.1 g∙m−2∙yr−1, respectively. Our analyses suggest that piscivore production is constrained by food web structure. Bloater, which comprise 72% of planktivore biomass, make up less than 20% of salmon diets. We es...

Effect of iron on productivity and size distribution of Antarctic phytoplankton

Abstract: In shipboard experiments, addition of Fe to samples from Antarctic shelf waters or from deep waters close to the shelf break did not have any detectable effect on phytoplankton populations. Fe addition to pelagic waters, however, increased Chl a concentrations by a factor of 4-7 times during l-2 weeks of incubation and also resulted in a shift from a nanoplankton-dominated population to one dominated by microplankton. If these shipboard experimental results are extrapolated to in situ results following enrichment of Antarctic pelagic waters with Fe, there may be some mitigation of the greenhouse effect caused by elevated CO, concentrations in the atmosphere. Not only would the rate of primary production increase, but also the percentage of primary production that is exported to deep water might be increased because of Fe favoring the growth of microplankton.

Recruitment from pelagic to early benthic phase in lobsters Homarus americanus

Abstract: The abundance of neustonic postlarvae and newly recruited benthic lobsters Homarus americanus was measured for 2 yr along the central coast of Maine, USA. Postlarvae first appeared in significant numbers in late July, rapidly increased in early August to ca 10 to 20 (1000 m 2 ) ' (0.5 m depth)', then gradually declined throughout August and into early September. Late intermolt stages Dl to Ds characterized most of the neustonic population, with a shift in dommance to D3 in the wanner year (1990). Postlarval abundances differed little between years: the seasonally and spatially averaged daily abundances were 5.58 (1000 m2)-' (0.5 m)\"' Â 0.77 (SE) and 6.76 Â 1.92 in 1989 and 1990, respectively. In contrast, there was a statistically significant difference in benthic recruitment measured in September. New recruits, lobsters with carapace length 5 10 mm, averaged 1.66 md. m 2 Â 0.37 (SE) in 1989 and 0.79 Â 0.22 in 1990. Spatial patterns in recruitment density at the sampling sites were positively correlated among years (r2 = 0.82) and did not appear to be related to distribution patterns of postlarvae. It seems likely that habitat type substantially influenced benthic recruitment patterns. Moreover, local differences in recruitment density of the first cohort were conserved over the 2 seasons of observation, supporting earlier assertions that new recruits are relatively sedentary. We develop a simple diagnostic model of the pelagic-to-benthic transition in lobster recruitment that accounts for the increase in density by 2 orders of magnitude from postlarval to benthic (recruit) stage, and we use this model to suggest plausible rates of postlarval diving and settlement that describe average and sitespecific recruitment in the 2 years.

Estimation of phosphorus exchange between littoral and pelagic zones during nighttime convective circulation

Abstract: A fluorescent dye was used in Eau Galle Reservoir, Wisconsin, to examine nighttime convective circulation patterns and exchanges of P between the littoral and pelagic zones during 25–26 July and 12–13 September 1988. Cooler littoral bottom water moved into the pelagic zone at night as an interflow at 2.5 m in July and at 4.0 m in September. Warmer pelagic water moved into the littoral zone as a surface flow during both periods. The littoral zone exhibited high total P concentrations above the sediment in July, but much lower concentrations in September. Estimated net areal total P flux to the pelagic zone was 1.8 mg m−2 d−1 in July but only 0.2 mg m−2 d−1 in September. Net areal total P flux in July accounted for 22% of the summer average, lakewide internal total P load to the reservoir.

Silurian Myodocopes: Pioneer pelagic ostracods and the chronology of an ecological shift

Abstract: . Analysis of all relevant palaeontological and global geological data strongly supports the notion that representatives of Silurian myodocope ostracods had pelagic lifestyles and habitats and that they may well be, within the Ostracoda, pioneer colonisers of such environments. Morphological evidence (from fossil and Recent myodocopes) combined with facies distributional and concomitant faunal evidence (from the Silurian of, for example, Britain, France, Czechoslovakia, Sardinia, Australia and China) endorses the idea that myodocope ostracods may have undergone a benthic to pelagic ecological shift during mid Silurian times. Lower Silurian myodocopes lived, with benthic associates, on well oxygenated shelves. Upper Silurian ostracods lived, typically with low diversity, largely pelagic faunas in outer shelf topographic lows or off-shelf basin slopes, and are characteristically associated with deposits which are in part suggestive of lowered oxygen levels or even anoxic conditions. A pre-adaptation for swimming may have allowed Silurian myodocopes to respond to environmental forcing (negative oxygen levels; positive trophic and nutrient incentives; rises in sea levels) by migrating, through time, up the water column.

Comparison of long-term trends in benthic and pelagic communities of the North Sea

Abstract: Species abundance data for benthic communities, collected during 1971–1988 off Northumberland on the north-east coast of England (western North Sea) and the Skagerrak (eastern North Sea), and pelagic data collected in corresponding areas during 1958–1988 by the Continuous Plankton Recorder (CPR) survey have been subjected tomulti-dimensional scaling ordination Changes in community structure at the two benthic stations show a high degree of similarity, characterised by a transition in the late 70s Similarly, there is a less marked transition between the 70s and 80s in pelagic community structure from the eastern North Sea but pelagic data from the western North Sea shows no discernible patterns Benthic community biomass data, available only for the Skagerrak station, enabled abundance/biomass comparison curves to be constructed which demonstrated a change from un-disturbed communities in the early and mid 70s to moderately disturbed communities in the late 70s and 80s North Sea macrofaunal abundance, and, in the Skagerrak, macrofaunal biomass appear to co-vary with phytoplankton colour and total zooplankton abundance, although there are insufficient data for statistical testing A number of factors including eutrophication and/or pollution may be responsible for these changes in community structure

Role of Cephalopods in the diet of the swordfish, Xiphias gladius, from the eastern Mediterranean Sea

Abstract: The gastric contents of 38 swordfish, Xiphias gladius, from the eastern Mediterranean Sea were examined to study the cephalopod component. Cephalopods were the most abundant prey items (their coefficient of prey frequency = 85.7%). Remains of 20 I cephalopod specimens were found, including loose hard parts, e.g., beaks, gladii, and lenses. Cephalopods belonged to eleven species, all pelagic. Todarodes sagittatus was the predominant food item. Next most abundant cephalopods were Ancistroteuthis /ichtensteinii and Heteroteuthis dispar. All other cephalopod species were ingested occasionally by swordfish. Based on the analyses of the loose beaks mantle length estimations were made and histograms oflower rostral length distribution were generated for T. sagittatus and A. /ichtensteinii. Swordfish are efficient collectors of otherwise rare pelagic animals. Based on the analysis of gastric contents, four cephalopods were recorded for the first time from the area: H. dispar, Onychoteuthis banksii, Histioteuthis bonnellii, Ancistrocheirus lesueurii. Some reflections were made about the local swordfish fishery and the exploitability of pelagic cephalopod stocks. Swordfish, Xiphic.rs gladius Linnaeus, 1758 (Osteichthyes: Xiphiidae), a cosmopolitan teleost, is an opportunistic predator which mainly feeds on pelagic species of both vertebrates and invertebrates; the taxon mostly preyed upon is Cephalopoda (see historical resume and results in Toll and Hess, 1981). As noted by Toll and Hess (1981), however, most authors fail to give a complete list of teuthological prey items found in the gastric content of swordfish. Although much work has been done on several aspects of the natural history and fishery biology of X. gladius, a commercially important fish, only one brief note reports a detailed list offood items ingested by swordfish from the eastern Mediterranean Sea (Bello, 1985). This, however, was limited to the examination of prey specimens with soft tissue remains and neglected loose hard parts (gladii, lenses, and beaks). In the present work examination was conducted on the gastric contents of 38 swordfish collected in the Apulian seas (Italy, eastern Mediterranean); they include 16 specimens whose partial analysis was previously reported in Bello (1985). Attention was focused on cephalopods, which represent the most abundant and diverse prey. All types of cephalopod remains were taken into account, including hard parts. The study of swordfish gastric content proved useful in gathering information on the ecology and fishery biology of both swordfish and cephalopods. Additions were made to the knowledge of pelagic cephalopod distribution in the eastern Mediterranean Sea, which is still poorly known (Mangold and Boletzky, 1988). MATERIALS AND METHODS The gastric content analysis was conducted on 38 specimens of Xiphias gladius collected by drifting longiine in the south Adriatic Sea, north Ionian Sea, and the Gulf of Taranto (inlet of the Ionian) (Fig. I), during the 1984 to 1986 swordfish fishery seasons. Swordfish ranged in length from 0.80 to 1.75 m (lower jaw to fork). Table I summarizes the swordfish capture data. The fishing season extends from late Spring (May-June) to early Autumn (October-November), with exact dates dependent on swordfish migration and prevailing meteorological conditions. The bottom depth in the fishing zone usually ranges from 500 m to the maximum depth in the Adriatic

Littoral-pelagic phosphorus dynamics during nighttime convective circulation

Abstract: Littoral phosphorus dynamics and P exchanges between littoral and pelagic zones, driven by nighttime convective circulation, were examined in Eau Galle Reservoir, Wisconsin, during summer 1989. Rates of P release from littoral sediments, measured in laboratory incubation systems, increased linearly with pH under both aerobic and aniaerobic-conditions. Estimated rates of P release from littoral sediments, based on field measurements of pH and oxygen, averaged 3.6 mg m-* d-l under aerobic conditions and were in close agreement with independently determined diffusional fluxes across the littoral-sediment interface. Marked vertical gradients in P developed during summer in bottom waters of the littoral zone, The: littoral zone cooled more rapidly at night than the pelagic zone on 72% of midsummer days (June-August), promoting nighttime convective circulation. On the basis of patterns of dye dispersion dluring these periods of circulation, cooler littoral bottom water moved into the pelagic zone as an interfiow confined to the base of the epilimnion, while warmer pelagic water moved into the littoral zone as a surface flow. Hourly volumetric flow rates were linearly related to mean negative horizontal temperature gradients that developed at night. Net total P flux from the littoral to the pelagic zone ranged from 0.12 to 0.43 mg mm2 h-’ (avg. 0.22) for the summer. Nighttime convective circulation seems to be an important mechanism for movement of littoral P to the pelagic zone in this reservoir,