Diel Depth Research Articles

Striped marlin in their Pacific Ocean milieu: Vertical movements and habitats vary with time and place

Striped marlin, Kajikia audax, a top bycatch of the longline fishery, has been designated as being in overfished condition in the Western and Central North Pacific, and overfishing is still occurring. This prompts an urgent need to devise conservation and management measures based on the best, current information on the biology and ecology of this species. Despite decades of conventional tagging around the Hawaiian waters, ecological research on striped marlin in the Central North Pacific has been lacking since 2005, and little is known about striped marlin’s vertical habitat, diving behavior and bycatch vulnerability in this area. To address this knowledge void, 31 popup satellite archival tags (4 X-Tags; Microwave Telemetry, Inc. and 27 MiniPATs; Wildlife Computers Inc.) were deployed on striped marlin (138-192 cm eye fork length) between 2016 and 2019 via the Hawaii-based longline fleet. Transmitted time series records revealed striped marlin spent 38 and 81% of their day and night in the top 5 m, with median daytime and night depths of 44 m and 2 m, respectively. Temperatures experienced were 23.3°C, daytime median, and 24.6°C, nighttime median, to a minimum of 7.6°C at the deepest depth logged, 472 m. Striped marlin exhibited distinct swimming behaviors, including diel depth distributions, excursions around the top of the thermocline, and extended time at the surface, most likely reflecting the dynamic biophysical environment and intrinsic life history of this highly migratory predator. High post-release survivorship (86%) in tagged striped marlin, and their predominant use of the sea surface and mixed layer indicate that live release measures can be a viable bycatch reduction strategy.

A trait‐based carbon export model for mesopelagic fishes in the Gulf of Mexico with consideration of asynchronous vertical migration, flux boundaries, and feeding guilds

AbstractFish‐mediated carbon export provides a significant proportion of the biological carbon pump in oligotrophic regions. Bioenergetic models estimate this carbon transport, but many lack species‐specific traits and no carbon export model has been developed in the mesopelagic Gulf of Mexico. Intensive mesopelagic sampling efforts in the northern and eastern Gulf of Mexico have provided high‐resolution information regarding community composition, species' vertical migratory characteristics, diel depth occupancies, and diets. A stochastic, individual‐based model was developed for deep‐pelagic fishes in the northern Gulf of Mexico to estimate bioenergetic rates and carbon export fluxes. Fishes that ate gelatinous zooplankton consumed more mass per body weight per day than predators of cephalopods and fishes, ostensibly to increase the throughput of prey with less carbon (gelata) or more refractory materials (Crustacea). A dynamic energy budget submodel indicated that during 81% of occurrences, asynchronous vertically migrating fishes rested for 1 d before migrating again, but migrations on successive days were possible. In terms of carbon export, myctophids and stomiids contributed greater than 53% and 12% of the active carbon flux for the entire assemblage in all scenarios. The assemblage‐wide carbon export rate driven by vertically migrating fishes was 0.14–0.72 mg C m−2 d−1, 61% of the ingested carbon by the assemblage. Incorporating species‐specific traits and individual variability in bioenergetic models allows for more complex research questions (e.g., the effect of feeding guilds and asynchronous migration on carbon export) compared to the carbon export models that otherwise assume all fishes within a functional group are equivalent.

Oxygen minimum zone: An important oceanographic habitat for deep-diving northern elephant seals, Mirounga angustirostris.

Little is known about the foraging behavior of top predators in the deep mesopelagic ocean. Elephant seals dive to the deep biota‐poor oxygen minimum zone (OMZ) (>800 m depth) despite high diving costs in terms of energy and time, but how they successfully forage in the OMZ remains largely unknown. Assessment of their feeding rate is the key to understanding their foraging behavior, but this has been challenging. Here, we assessed the feeding rate of 14 female northern elephant seals determined by jaw motion events (JME) and dive cycle time to examine how feeding rates varied with dive depth, particularly in the OMZ. We also obtained video footage from seal‐mounted videos to understand their feeding in the OMZ. While the diel vertical migration pattern was apparent for most depths of the JME, some very deep dives, beyond the normal diel depth ranges, occurred episodically during daylight hours. The midmesopelagic zone was the main foraging zone for all seals. Larger seals tended to show smaller numbers of JME and lower feeding rates than smaller seals during migration, suggesting that larger seals tended to feed on larger prey to satisfy their metabolic needs. Larger seals also dived frequently to the deep OMZ, possibly because of a greater diving ability than smaller seals, suggesting their dependency on food in the deeper depth zones. Video observations showed that seals encountered the rarely reported ragfish (Icosteus aenigmaticus) in the depths of the OMZ, which failed to show an escape response from the seals, suggesting that low oxygen concentrations might reduce prey mobility. Less mobile prey in OMZ would enhance the efficiency of foraging in this zone, especially for large seals that can dive deeper and longer. We suggest that the OMZ plays an important role in structuring the mesopelagic ecosystem and for the survival and evolution of elephant seals.

Seasonal and Diel Bathythermal Distributions of Lake Whitefish in Lake Huron: Potential Implications for Lake Trout Bycatch in Commercial Fisheries

AbstractDepths and temperatures recorded during 2003–2005 by archival tags implanted in Lake Whitefish Coregonus clupeaformis and in Great Lakes origin (GLO) and New York Finger Lakes origin (FLO) strains of Lake Trout Salvelinus namaycush were used to compare seasonal diel depth and temperature distributions in Lake Huron. Seasonal depth distributions were examined to determine if species differences could be exploited to reduce bycatch of Lake Trout in commercial Lake Whitefish gill‐net fisheries. Both GLO and FLO Lake Trout used deeper and colder waters than Lake Whitefish during daylight and dark. Temperature differences between species were greatest during periods of stratification when behavioral thermoregulation was possible. Other than during spawning periods, the greatest depth and temperature separation occurred in late July between FLO Lake Trout (37.0 m; 6.5 ºC) and Lake Whitefish (25.1 m; 10.2 ºC). If maximum depths of gill nets were regulated seasonally to between 25 and 35 m, ≥ 50% of the Lake Whitefish population would be vulnerable while avoiding 89% or more of Lake Trout. Lake Trout percentages targeted under such regulations would be lowest in late July (GLO = 8% and FLO = 11%) and early August (GLO = 4% and FLO = 7%). However, archival tags measure fish depth and not location or bottom depth, and modal depths of gill‐net effort for Lake Whitefish (38 to 57 m) exceeded those from the Lake Whitefish archival tag data. This discrepancy suggests that many Lake Whitefish might be pelagic above the reach of bottom‐set gill nets, so depth restrictions could be less effective in reducing Lake Trout bycatch than suggested by our data. A further implication is that use of gill nets suspended above the bottom to target pelagic Lake Whitefish could also reduce bycatch while potentially increasing Lake Whitefish harvest.Received August 19, 2015; accepted February 21, 2016 Published online June 28, 2016

Vertical and horizontal movements of a silvertip shark (Carcharhinus albimarginatus) in the Fijian archipelago

Despite widespread distribution and occurrence in the global shark fin trade, information regarding fundamental biology of the silvertip shark, especially vertical and horizontal movement data, is sparse. Its habitat-faithful nature, confined geographical populations, and life history suggest silvertip sharks are vulnerable to overexploitation, particularly in heavily longlined regions with limited offshore management, such as the Fijian archipelago. Satellite telemetry identified diel depth, temperature, and vertical habitat use differences, previously unreported, in a tracked individual. Although the individual occupied shallower mean depths at night, nighttime excursions below 200 m were recorded. Net displacement from tagging location was estimated <18 km. Diel depth differences and expanded daytime depth use could indicate foraging behavior, routine predator avoidance, or temperature selection. Typically, pelagic species are more heavily impacted by commercial longline fishing than reef-associated species, particularly in regions such as Fiji, where reefs are locally managed. The apparent mix of reef-associated and pelagic behaviors, coupled with high levels of commercial longlining in the region make the silvertip shark especially vulnerable to exploitation. Limited sample size prevented drawing any conclusions about the species, but these preliminary results suggest in order to be effectively managed the silvertip shark warrants additional movement studies and stock assessment surveys throughout it range.

Spatial Dynamics and Expanded Vertical Niche of Blue Sharks in Oceanographic Fronts Reveal Habitat Targets for Conservation

Dramatic population declines among species of pelagic shark as a result of overfishing have been reported, with some species now at a fraction of their historical biomass. Advanced telemetry techniques enable tracking of spatial dynamics and behaviour, providing fundamental information on habitat preferences of threatened species to aid conservation. We tracked movements of the highest pelagic fisheries by-catch species, the blue shark Prionace glauca, in the North-east Atlantic using pop-off satellite-linked archival tags to determine the degree of space use linked to habitat and to examine vertical niche. Overall, blue sharks moved south-west of tagging sites (English Channel; southern Portugal), exhibiting pronounced site fidelity correlated with localized productive frontal areas, with estimated space-use patterns being significantly different from that of random walks. Tracked female sharks displayed behavioural variability in diel depth preferences, both within and between individuals. Diel depth use ranged from normal DVM (nDVM; dawn descent, dusk ascent), to reverse DVM (rDVM; dawn ascent, dusk descent), to behavioural patterns where no diel differences were apparent. Results showed that blue sharks occupy some of the most productive marine zones for extended periods and structure diel activity patterns across multiple spatio-temporal scales in response to particular habitat types. In so doing, sharks occupied an extraordinarily broad vertical depth range for their size (1.0–2.0 m fork length), from the surface into the bathypelagic realm (max. dive depth, 1160 m). The space-use patterns of blue sharks indicated they spend much of the time in areas where pelagic longlining activities are often highest, and in depth zones where these fisheries particularly target other species, which could account for the rapid declines recently reported for blue sharks in many parts of the world's oceans. Our results provide habitat targets for blue shark conservation that may also be relevant to other pelagic species.

Use of satellite tags to reveal the movements of spiny dogfish Squalus acanthias in the western North Atlantic Ocean

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 418:249-254 (2010) - DOI: https://doi.org/10.3354/meps08821 NOTE Use of satellite tags to reveal the movements of spiny dogfish Squalus acanthias in the western North Atlantic Ocean James A. Sulikowski1,*, Benjamin Galuardi2, Walter Bubley3, Nathan B. Furey1, William B. Driggers III4, G. Walter Ingram Jr.4, Paul C. W. Tsang3 1Marine Science Center, University of New England, 11 Hills Beach Rd, Biddeford, Maine 04005, USA 2Large Pelagics Research Center, Room 177, Spaulding Hall, University of New Hampshire, 46 College Rd, Durham, New Hampshire 03824, USA 3Department of Molecular, Cellular and Biomedical Sciences, Kendall Hall, University of New Hampshire, 129 Main Street, Durham, New Hampshire 03824, USA 4National Marine Fisheries Center, Southeast Fisheries Science Center, Mississippi Laboratories, PO Drawer 1207, Pascagoula, Mississippi 39568, USA *Email: jsulikowski@une.edu ABSTRACT: The use of popup satellite archival transmitting tags (PSAT) has helped to reveal horizontal and vertical movements of large, highly migratory fishes to assist in the management of their stocks. We tested the efficacy of a significantly smaller satellite tag, the X-tag, to track the movements of 3 spiny dogfish Squalus acanthias in the western North Atlantic Ocean. Tag retention rates for this study ranged between 65 and 189 d. The sharks occupied waters with temperatures ranging from 5.2 to 14.9°C and had estimated average horizontal speeds between 1.36 and 2.49 km h–1. The sharks initially moved east into offshore waters after tagging in the Gulf of Maine and then into southern waters off the coast of New Jersey, USA. Diel depth patterns indicated that each shark was equally active during both day and night. Our results suggest that spiny dogfish are more mobile, both vertically and horizontally, than previously thought and that smaller PSATs can provide useful information on the movement patterns of a relatively small species of shark. KEY WORDS: Spiny dogfish · Movement · Depth preferences · Tracking Full text in pdf format PreviousNextCite this article as: Sulikowski JA, Galuardi B, Bubley W, Furey NB, Driggers WB III, Ingram GW Jr, Tsang PCW (2010) Use of satellite tags to reveal the movements of spiny dogfish Squalus acanthias in the western North Atlantic Ocean. Mar Ecol Prog Ser 418:249-254. https://doi.org/10.3354/meps08821Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 418. Online publication date: November 18, 2010 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2010 Inter-Research.

Insights into catch-and-release survivorship and stress-induced blood biochemistry of common thresher sharks ( Alopias vulpinus) captured in the southern California recreational fishery

The common thresher shark ( Alopias vulpinus) is the focus of a popular southern California recreational fishery that typically captures individuals by hooking them in the caudal fin. This technique reduces the ability for forward locomotion and the capacity for ram ventilation. This study assessed the post-capture survivorship of tail-hooked adult and sub-adult common thresher sharks using pop-up satellite archival tags (PSATs) and quantified physiological indicators of capture stress in the blood. Survival of the acute effects of capture was determined from the depth and temperature records of 10-day PSAT deployments. Survivorship estimates were based on 19 common thresher sharks [160–221 cm fork length (FL); ∼67–151 kg] captured in southern California from 2007 to 2009 using recreational stand-up tackle (36 kg). Five mortalities were observed over the course of the study resulting in an overall post-release mortality estimate of 26%. All mortalities occurred in large individuals (≥180 cm FL) with fight times ≥85 min. The archived depth and temperature data from surviving sharks resembled those of previous common thresher movement studies with a diel depth distribution predominantly within the uniformed temperature surface layer. Capture induced stress parameters measured from the blood of eight additional common thresher sharks that were not tagged revealed plasma lactate and hematocrit levels that were significantly elevated with increased fight time. Similarly, all thresher sharks showed heightened heat shock protein 70 ( hsp 70) values relative to those obtained from blood that was allowed to recover in vitro for 24 h. Collectively, our findings indicate that large tail-hooked common thresher sharks with prolonged fight times (≥85 min) exhibit a heightened stress response which may contribute to an increased mortality rate. These results suggest that for larger individuals the current caudal-based capture methods used in the California recreational fishery may not be suitable for an effective catch-and-release based conservation strategy.

Diel depth distributions of microbenthos in tidal creek sediments: high resolution mapping in fluorescently labeled embedded cores

Intertidal sediments experience substantial changes in temperature, salinity, and solar irradiation over short time periods. We applied the Fluorescently Labeled Embedded Core (FLEC) technique to map distribution patterns of microbenthos in tidal creek sediments. Our aims were to determine if micro-scale distributions varied over the course of a day and to test the null hypothesis that microbenthos are randomly distributed. Eight samples were collected at 3 h intervals from an intertidal sandflat on Sapelo Island, Georgia, USA. Cores were incubated with CellTracker Green (CMFDA, hereafter, CTG), a fluorogenic compounds that accumulates in metabolically active cells. Cores were embedded with epoxy and examined with laser scanning confocal microscopy. Image analysis was used to map the vertical locations of active microbenthos, which in these sediments consisted of benthic microalgae (BMA), ciliates, and flagellates. Microbenthos were abundant over the entire depth profiled (2 cm), although O2 microelectrode profiles indicate that only the top 3 mm of sediment was oxygenated during high light (1,000 μE m−2 s−1). More than 91% of organisms mapped were <22 μm in diameter and, based upon size and cell appearance, were BMA. Microbenthos accumulated in the top 1 mm at 0800 and 1100 h, corresponding to both low tide and high solar irradiation. This pattern conforms to BMA migratory rhythms determined by other methods. The standardized Morisita’s Index of dispersion determined that CTG-positive objects were significantly clumped at all time points and at each of the three spatial scales examined. This clumping pattern likely results from the heterogeneous distribution of resources, such as prey items for phagotrophs and dissolved nutrients or growth substrates for autotrophs or heterotrophs.

Movement patterns and nursery habitat of juvenile thresher sharksAlopias vulpinusin the Southern California Bight

We investigated the potential use of open coastal habitat over the continental shelf as a nursery area for the common thresher shark Alopias vulpinus. Seven juvenile threshers were tracked using acoustic telemetry to determine their movement patterns and nursery habitat in the Southern California Bight (SCB). Tracked sharks occupied waters over the continental shelf 87% of the time. These waters had an average (± SD) sea surface temperature of 18.8 ± 1.6°C and chlorophyll concen- trations that were an order of magnitude higher than in adjacent waters offshore of the continental shelf. Tracked sharks had a mean rate of movement of 1.63 ± 0.56 km h -1 , and some sharks exhibited high site fidelity. The vertical distribution of juvenile threshers was generally limited to the upper 20 m of the water, and most sharks showed diel depth distribution patterns, with daytime depths significantly greater than nighttime depths. An analysis of SCB commercial fishery observer data confirms that juvenile common threshers are most frequently captured over the continental shelf. This region appears to provide juvenile threshers with ample food resources and reduced predation risk relative to adult habitat, and partially satisfies more quantitative nursery area criteria recently established in the literature.

First results from satellite-linked archival tagging of porbeagle shark, Lamna nasus: Area fidelity, wider-scale movements and plasticity in diel depth changes

Understanding the habitat preferences of large marine vertebrates has only recently become tractable with the widespread availability of satellite telemetry for monitoring movements and behaviour. For many species with low population abundances, however, little progress has been made in identifying space use patterns. The endothermic porbeagle shark, Lamna nasus, has declined in the North Atlantic due to severe fishing pressure, with little evidence of recovery. One potential factor exacerbating population decline is area fidelity to coastal waters where fisheries are intensive. We tested for short-term area fidelity by attaching pop-up satellite-linked archival transmitters to four porbeagles in summer 2007, resulting in 175 days total tracking time covering an estimated 10,256 km distance. Throughout July and August the sharks occupied localised areas (8,602 – 90,153 km 2) within the Celtic Sea, between the south-west UK, south-west Wales and southern Ireland. Only one shark was tracked into the autumn, when it moved into deep water off the continental shelf, then north towards colder latitudes. Sharks occupied a broad vertical depth range (0 – 552 m) and water temperatures (9° - 19 °C). Dives were made frequently from the surface to near the seabed in shelf areas, however, in shelf edge habitats extended periods of time were spent at depths > 300 m. Porbeagles showed considerable plasticity in diel depth changes within and between individuals and as a function of habitat type. In addition to no obvious day-night difference in depth occupation, some sharks showed reverse diel vertical migration (DVM) (dawn ascent – dusk descent) in well-mixed coastal waters whereas normal DVM (dawn descent – dusk ascent) characterised movements into deeper, thermally well-stratified waters. The variable behaviours may reflect the need for different search strategies depending on habitat and prey types encountered. These results show porbeagles are potentially vulnerable to fisheries throughout the summer when they aggregate, and that large scale movement across national boundaries identifies the need for international conservation measures.

Diving behavior and movements of juvenile hawksbill turtles Eretmochelys imbricata on a Caribbean coral reef

As historically abundant spongivores, hawksbill turtles Eretmochelys imbricata likely played a key ecological role on coral reefs. However, coral reefs are now experiencing global declines and many hawksbill populations are critically reduced. For endangered species, tracking movement has been recognized as fundamental to management. Since movements in marine vertebrates encompass three dimensions, evaluation of diving behavior and range is required to characterize marine turtle habitat. In this study, habitat use of hawksbill turtles on a Caribbean coral reef was elucidated by quantifying diel depth utilization and movements in relation to the boundaries of marine protected areas. Time depth recorders (TDRs) and ultrasonic tags were deployed on 21 Cayman Islands hawksbills, ranging in size from 26.4 to 58.4 cm straight carapace length. Study animals displayed pronounced diel patterns of diurnal activity and nocturnal resting, where diurnal dives were significantly shorter, deeper, and more active. Mean diurnal dive depth (±SD) was 8 ± 5 m, range 2–20 m, mean nocturnal dive depth was 5 ± 5 m, range 1–14 m, and maximum diurnal dive depth was 43 ± 27 m, range 7–91 m. Larger individuals performed significantly longer dives. Body mass was significantly correlated with mean dive depth for nocturnal but not diurnal dives. However, maximum diurnal dive depth was significantly correlated with body mass, suggesting partitioning of vertical habitat by size. Thus, variable dive capacity may reduce intraspecific competition and provide resistance to degradation in shallow habitats. Larger hawksbills may also represent important predators on deep reefs, creating a broad ecological footprint over a range of depths.

Comparison of the Development and Function of the Swimbladder ofBrevoortia tyrannus(Clupeidae) andLeiostomus xanthurus(Sciaenidae)

The swimbladders of physostomous Brevoortia tyrannus (Atlantic menhaden) and physoclistous Leiostomus xanthurus (spot) develop as a simple evagination of the larval foregut. The swimbladder of L. xanthurus develops earlier (at yolk and oil globule depletion when larvae are two days posthatch and ∼2 mm notochord length) than it does in B. tyrannus (well after yolk and oil depletion when larvae are 12 days and ∼8 mm). The larvae of each species initially inflate the swimbladder by ascending to the surface and forcing atmospheric gas through a pneumatic duct into the swimbladder lumen. Larval B. tyrannus modulate swimbladder volume and buoyancy within a diel cycle by inflation with atmospheric gas passed through a persistent pneumatic duct and deflation by diffusion through the swimbladder wall along with expulsion through the anus and mouth. This capacity of swimbladder volume modulation and buoyancy compensation corresponds with the average vertical distribution of larvae in the sea, where larval B. tyrannus occupy the upper water column. The pneumatic duct of L. xanthurus atrophies after 20 days and has no discernable lumen after 40 days, the beginning of metamorphosis when larvae are ∼7 mm standard length. The mucosal epithelium of the swimbladder of L. xanthurus acquires cytological characteristics of adult gas-gland tissue soon after initial inflation when larvae gain the capability of gas secretion. A plexiform rete mirabile does not form until metamorphosis. The swimbladder of larval L. xanthurus functions by providing neutral buoyancy with low amplitude diel depth changes in swimbladder volume. In the sea, larval L. xanthurus occupy primarily mid- and deep depths.

An evaluation of factors affecting vertical distribution among recruits of Calanus finmarchicus in three adjacent high-latitude localities

The vertical distributions of populations of Calanus finmarchicus are described in three different fjord areas near Tromse, northern Norway during May 1986. These localities (Malangen, Grotsund and Balsfjorden) had characteristic differences in temperature, phytoplankton and population density of copepods. They probably are representative annual situations during the spring and summer period for coastal and fjord areas in northern Norway. Copepodite stage I and II C. finmarchicus are found in the surface waters (0–30 m) during a 24 h cycle, while the other stages appear to have a different diel depth distribution in Malangen. Pronounced differences in the depth distribution of the various copepodite stages and adult females were found in Grotsund and Balsfjorden during the same period of the day on 20 and 21 May. The tendency for vertical overlap among CI–CV was clearly less pronounced in an environment with low phytoplankton standing stock and high population density of copepods. The patterns of vertical distribution are analysed by multidimentional scaling (MDS) and it is evident that the distribution pattern of C. finmarchicus is different at each locality. These preliminary results, are discussed in relation to ontogenetic vertical migration and aspects of resource partitioning and the possible importance of vertical separation for reducing competitive interactions between the different life stages of C. finmarchicus.

A seasonal sequence of diel distribution patterns for the planktonic dinoflagellate Ceratium hirundinella in a eutrophic lake

SUMMARY. 1. Diel depth distribution patterns of Ceratium hirundinella were studied during eleven sampling periods, covering the seasonal growth cycle. They were shown to result from short‐term periodic or non‐periodic external factors, endogenous responses of the alga, and interactions between these.2. Diel variations in wind stress resulted in the net transport of algae into or out of the sampling position due to lateral water movements. A progressive surface accumulation of Ceratium, leading to a 3‐fold increase in cell numbers at the sampling site over a 24 h period, was due to wind‐induced upwelling of deeper cells. Near‐surface accumulation of Ceratium on a completely overcast afternoon, similar to that associated with migrations, was attributable to advection.3. Under sufficiently calm conditions, depth‐differentiation of Ceratium was regulated by its vertical swimming movements with a diel periodicity. Migration patterns observed in earlier work were confirmed and extended. The alga migrated towards the surface during the daytime and downward during the night; this rhythm had an endogenous component.4. At high surface illumination, the alga retreated from the surface and formed discrete sub‐surface maxima; at low irradiance Ceratium showed positive phototaxis and concentrated near the surface. Under either condition of irradiance Ceratium actively aggregated at depths associated with irradiance levels of about 125–155 μEinsteins m −2 s −1. Downward movement was restricted by anoxic conditions and possibly by thermal/ density gradients. However, given sufficient light penetration, dissolved oxygen and nutrient availability, the thermal density gradient does not apparently eliminate downward movement by Ceratium.

Diel variations of selected physico-chemical parameters in Lake Kissimmee, Florida

Every two months, diel depth profiles were made of dissolved oxygen, pH, temperature and conductivity at two stations in Lake Kissimmee, Florida, from July 1974 through June 1975. Results suggest that stratification does not occur in the large (137 km2), shallow (mean depth 2.5 m) lake, though steep vertical gradients in these parameters may develop. The data also suggest that when these gradients do occur, they rapidly degenerate at night or under slight wind stress. Since many Florida lakes are relatively shallow (mean depth < 5 m) and topography offers little obstruction to wind action, it is believed that the holomictic condition observed in Lake Kissimmee may be typical of the state's lakes, particularly those in the central portion of the peninsula.