Zooplankton Layers Research Articles

Multiple sampling methods to develop indices of mid‐trophic levels abundance in open ocean ecosystems

AbstractMid‐trophic level (MTL) organisms play a key role in the sub‐Antarctic ecosystem food web, linking primary producers and tertiary consumers transferring energy across trophic levels. In this region, the relative abundance of MTLs has been monitored using single‐frequency acoustic data collected opportunistically in a time series of trawl surveys carried out in the austral late spring/early summer. To advance this approach, we developed a methodology that enabled us to disaggregate multifrequency acoustic data into three different MTL groups to assess their temporal and spatial patterns: mesopelagic fish, gelatinous zooplankton (excluding pyrosomes), and euphausiids. We used acoustic data and biological sampling information collected with two midwater trawl nets from a dedicated voyage, to train a classification tree algorithm. We complemented these data with footage from an in‐trawl camera system that gave us new insights on vertical distribution and presence of pelagic taxa. The testing dataset used to implement our classification algorithm was a time series of acoustic data from the sub‐Antarctic trawl survey. This approach was successful at disaggregating acoustic data and provided useful information to study the relative abundance and temporal and spatial patterns of pelagic taxa. Our analysis revealed the presence of dense shallow layers of gelatinous zooplankton which showed a statistically significant increasing trend over the time series. Although we were unable to detect pyrosomes acoustically, we encountered large biovolumes of these organisms during our biological sampling, which was supported by the in‐trawl camera system.

A deep dive into the ecology of fear

The largest migration on Earth occurs on a daily basis in the ocean as creatures move up near the surface during the night to feed and descend to the relative safety of the dark depths during the day. This phenomenon is thought to reflect feeding opportunities and predator avoidance according to light levels, but the dynamics of this have rarely been studied. Samuel Urmy and Kelly Benoit-Bird from the Monterey Bay Aquarium Research Institute, USA, inspired by the originally terrestrial concept of ‘landscapes of fear’, sought to investigate if the fear of predation shapes the ecological process of daily migrations.Urmy and Benoit-Bird used an upwards-facing echosounder, which produces sonar pulses and records the returning echoes to detect and visualize schools of fish, squid and plankton, to provide information about their daily vertical movement patterns. Additionally, an underwater microphone next to the echosounder listened for the hunting sounds made by nearby toothed whale predators, such as Risso's dolphins (Grampus griseus) and Pacific white-sided dolphins (Lagenorhynchus obliquidens). Recording the echolocation clicks gave the researchers an indication of the presence of a predator, allowing them to investigate the interactions between the predators and the prey in their vicinity. Successfully collecting data beneath almost 1 km of seawater for more than a year is no easy feat, so the team sent down an unmanned robot to survey the area with video to visually confirm the identities of the different species that they were picking up with the echosounder.Scrutinizing a whole year of deep-sea data, the researchers observed many examples of prey taking evasive action to avoid predators. In one example, a bout of dolphin echolocation clicks triggered a school of fish to cluster together and dive for safety moments later. While such a diving response physically increases the distance between the fish and the predator, the fish's vertical plunge also results in weaker echoes returning from the vulnerable fish, reducing the opportunity for the hunting predator to successfully home in on their fishy prey. In addition, the team could use the distinctive echo signatures produced by different species – such as larger Pacific hake (Merluccius productus) and smaller anchovies (Engraulis mordax) – to provide a best guess for which species were appearing in the echosounder traces when video data were not available. This was especially useful for identifying instances of predation that did not involve an echolocating species, such as an occasion when a school of predatory fish approached a cloud of zooplankton and the tiny creatures dived to avoid the hunters. The scientists also discovered that it sometimes took clusters of zooplankton weeks to return to their usual depth after an encounter with a hungry predator. This shaping of a prey animal's behaviour and movement, even in the absence of a predator, provides evidence that the concept of the ‘ecology of fear’ is indeed applicable in the 3D marine realm and complements recent research on marine ‘soundscapes of fear’, where prey fall silent to avoid detection by predators that could be nearby.The vertical movement of animals in the ocean plays a major part in nutrient recycling, and Urmy and Benoit-Bird demonstrate that the lasting behavioural responses of prey to hunting predators potentially has a major influence on this flow of nutrients. The effect of predators on the behaviour of their prey can thus be more lingering than we had thought. Zooplankton layers in the open ocean are therefore not just governed by forces of nature or their particular lifestyles, but by the ecology of fear too.

Fear dynamically structures the ocean’s pelagic zone

Fear of predation can have wide-ranging ecological effects.1-4 This is especially true in the ocean's pelagic zone, the Earth's largest habitat, where vertical gradients in light and primary productivity force numerous taxa to migrate vertically each night to feed at the surface while minimizing risk from visual predators.5-7 Despite its importance and the fact that it is driven by spatial differences in perceived risk,8 diel vertical migration (DVM) is rarely considered within the "landscape of fear"3,8,9 framework.10 It is also far from the only such process in the pelagic zone. We used continuous, year-long records from an upward-looking echosounder and broadband hydrophone at a cabled observatory off Central California, USA, to observe avoidance reactions by several groups of pelagic animals to the presence of their predators. As expected, vertical migration was ubiquitous, but we also observed behaviors at shorter and longer timescales that were best explained by fear of predation. The presence of foraging odontocetes induced immediate diving behavior in mesopelagic sound-scattering layers, and schools of epipelagic fishes induced similar reaction in layers of zooplankton and mesopelagic micronekton. At longer timescales, the presence of fish schools significantly deepened vertical migration, rearranging life throughout the water column. We argue that behavioral reactions to predation risk are common in the pelagic zone at a range of spatiotemporal scales and that our understanding of food webs and biogeochemical cycling in this immense biome will be incomplete unless we account for fear.

Internal solitary waves on the NW African shelf: A heuristic approach to localize diapycnal mixing hotspots

Turbulent mixing on continental shelves plays roles in the structure and dynamics of marine ecosystems, nutrient cycling, primary production and dispersion of pollutants. Describing and understanding internal wave dynamics enables improved mapping of mixing over continental shelves, especially in complex environments where many processes may interact, such as in upwelling systems. This paper describes internal wave propagation and dissipation in the Moroccan upwelling system using a comprehensive set of hydrographic observations made during two in situ surveys. The acoustic backscatter of the echosounder is shown to be a rapid and convenient survey tool for detecting internal solitary waves (ISWs) on large spatial scales, through the high-frequency oscillation of the zooplankton layer during nighttime conditions. Along ISW trains, enhanced diapycnal mixing episodes are observed with increased dissipation coefficients in the thermocline (O(10-7 W kg-1)), associated with overturning up to 6 m. Mixing due to internal wave soliton packets shows large spatial and temporal variability, but contributes to the overall mixing processes and is responsible for large intermittent variations in the thermocline position. The joint use of a multifrequency echosounder and a current profiler ADCP allows mixing to be quantified (via the Richardson number) on large spatial scales. Validation and use of this method in other coastal regions could be useful to determine regional mixing parameterization in numerical models.

Bioluminescence of ctenophores near the boundary of oxygen-depleted waters at the redoxcline of the Black Sea.

Vertical distribution of ctenophores near the boundary of oxygen-depleted waters of the Black Sea redoxcline was studied by use of video observations with real-time water sampling, horizontal MultiNet towing, and soundings using bathyphotometers with simultaneous vertical plankton net sampling. The results of the study showed for the first time that the daytime accumulation of ctenophores above the upper boundary of the suboxic zone changes the biophysical properties of the medium, causing an increase in the daytime intensity of bioluminescence near the redoxcline. The dynamics of this glow is in antiphase to that in the surface layers, where it is associated with the bioluminescence of phytoplankton. Therefore, in the deep-sea areas, two types of bioluminescence peaks differ in the light generation sources: the nighttime glow of phytoplankton in surface layers and the daytime glow of zooplankton in layers of oxygen-depleted waters at the redoxcline. The discovery of this new phenomenon allows the use of bioluminescent methods for the rapid assessment of the depth of the daytime zooplankton layers for the subsequent hauls of plankton nets. This significantly expands the possibilities of studying the structure and functioning of the pelagic ecosystem of the Black Sea and other marine basins with a redoxcline.

Study of the Arctic mesopelagic layer with vessel and profiling multifrequency acoustics

The range limitation (>200 m) for high-frequency echosounders does not allow for complete multifrequency studies of the mesopelagic layers from vessel-mounted echosounders. The layers of mesopelagic fish and zooplankton in the Arctic region north of Svalbard (Spitsbergen) were studied using vessel-mounted echosounders and a profiling acoustic probe, using 38, 120, 200 and 333 kHz. Volume density estimates of mesopelagic fish have shown to be marginally higher with the probing system in relation with measured from the vessel-mounted echosounders at 38 kHz. This shows that the swimbladder resonance phenomenon is not occurring in low density layers with limited vertical migration. The use of the profiling probe allowed densities to be calculated with an in situ measured target strength (TS). In depths >200 m where high-frequency hull-mounted transducers cannot effectively reach, the profiling system measured a mixture of krill and amphipods down to 600 m. Vertical profiles of measured target categories, from vessel transducers and from the probing system are compared in relation to the biological sampling conducted during the survey. Profiling acoustics are shown to be a valuable tool to address some limitations in the current surveying methods for studying mesopelagic layers beyond the reach for high-frequency vessel-mounted systems.

Seasonal patterns in deep acoustic backscatter layers near vent plumes in the northeastern Pacific Ocean

We used moored 75 kHz acoustic Doppler current profilers (ADCPs) to examine seasonal cycles in zooplankton deep scattering layers (DSLs) observed below 1300 m depth at Endeavour Ridge hydrothermal vents. DSLs are present year-round in the lower water column near vent plumes. Temporal variations suggest passive, flow-induced displacements superimposed on migratory movements. Although the strongest DSLs are shallower than the neutrally buoyant plumes (1900–2100 m), anomalies also occur at and below plume depth. Upward movement from plume depth in the main DSL is evident in late summer/fall, resulting in shallower DSLs in winter, consistent with the timing of adult diapause/reproduction in upper-ocean migratory copepods. Movement from the upper ocean to plume depth coincides with pre-adult migration to greater depths in spring. Synchronous 20–40 d cycles in DSLs may account for patchiness in space and time of above-plume zooplankton layers observed in summer during previous net-sampling surveys, and suggests lateral and vertical migratory movements to counter current drift away from plume-derived food sources. Persistent near-bottom DSLs move vertically between the spreading plume and seafloor. Historical net data suggests that these are deep, resident fauna. Unlike upper ocean fauna, they seem to be advected considerable distances from the ridge axis, where they are evident as remnant scattering layers.

Hyperbenthic food-web structure in an Arctic fjord

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 603:29-46 (2018) - DOI: https://doi.org/10.3354/meps12713 Hyperbenthic food-web structure in an Arctic fjord Maeve McGovern1,2,3,4,6,*, Jørgen Berge2,3, Beata Szymczycha5, Jan Marcin Węsławski5, Paul E. Renaud1,2 1Akvaplan-niva, Fram Centre for Climate and the Environment, 9296 Tromsø, Norway 2University of Tromsø, 9027 Tromsø, Norway 3University Centre in Svalbard, 9171 Longyearbyen, Norway 4Nord University, Faculty of Biosciences and Aquaculture, Postbox 1490, 8049 Bodø, Norway 5Institute of Oceanology PAS, ul. Powstancow Warszawy 55, 81-712 Sopot, Poland 6Present address: Norwegian Institute for Water Research, Gaustadalleen 21, 0349 Oslo, Norway *Corresponding author: maeve.mcgovern@niva.no ABSTRACT: Current knowledge of the Arctic marine ecosystem is based primarily on studies performed during the polar day on the pelagic and benthic realms. Both the polar night and the hyperbenthic layer remain as substantial knowledge gaps in our understanding of the marine system at high latitudes. This study investigated the hyperbenthic food web in Kongsfjord, a high-latitude, ice-free fjord, in September 2014 and January 2015. The hyperbenthic food web was analyzed using a multi-biomarker approach including stable isotopes of carbon and nitrogen as well as fatty acid profiles of a variety of hyperbenthic taxa. While results suggested no difference in biomarker composition between September and January, they indicated a division in the sampled hyperbenthic species assemblage between pelagic and benthic feeders in both sampling periods. The presence of these 2 food-web pathways may have implications for maintaining higher trophic levels through the polar night, thereby enhancing stability in the Kongsfjord system. KEY WORDS: Hyperbenthos · Suprabenthos · Benthic-boundary layer zooplankton · Kongsfjord · Stable isotopes · Fatty acids · Benthic-pelagic coupling Full text in pdf format PreviousNextCite this article as: McGovern M, Berge J, Szymczycha B, Weęsławski JM, Renaud PE (2018) Hyperbenthic food-web structure in an Arctic fjord. Mar Ecol Prog Ser 603:29-46. https://doi.org/10.3354/meps12713 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 603. Online publication date: September 17, 2018 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2018 Inter-Research.

Fine-scale zooplankton diel vertical migration revealed by traditional net sampling and a Laser Optical Plankton Counter (LOPC) in Lake Ontario

Abstract Traditional net hauls used to monitor zooplankton population change in the Great Lakes do not provide information on vertical distribution – an important aspect of the availability of zooplankton to predators. During 2013, we combined net data with data from a Laser Optical Plankton Counter (LOPC) to track the vertical distribution of zooplankton in Lake Ontario in May, July, and September. Closing nets (64 μm mesh) sampled three strata at stations of north-south transects. For a 10 km segment of each transect in May and September, a LOPC was towed from the surface to 60 m depth. In May, biomass was highest in the epilimnion both day and night. Diacyclops thomasi dominated in the epilimnion while Limnocalanus macrurus dominated deeper. During strongly stratified conditions in July and September, closing net data confirmed that biomass was highest in the metalimnion during the day and in the epilimnion during the night. D. thomasi, Daphnia retrocurva, and L. macrurus were major components and vertical migrators. In May, LOPC data showed that biomass was highest in the upper 10 m of the epilimnion for day and night. In September, LOPC data showed two layers during the day, a narrow 5 m layer in the epilimnion at about 20 °C and a 15 m layer below the thermocline at 10 °C. The lower layer followed the deepening thermocline from west to east. At night, the LOPC detected most biomass throughout the epilimnion. The LOPC data confirmed diel vertical migration and detailed the fine-scale structure of zooplankton layers.

Seasonal and interannual changes in the sound scattering layer at deep-sea floor in the Amundsen Sea, Antarctica

Vertical migration of zooplankton is ubiquitous behavior in marine plankton community; however, seasonal and interannual behavior are little observed in the deep sea under seasonal varying sea ice. Here, the first evidence that sound scattering layers of zooplankton can support the knowledge for understanding the effect of climate change is presented, based on four-years acoustic backscattering strengths in the Amundsen Sea, Antarctica. Amundsen Sea is a biological hotspot region with the rapid oceanic melting of the ice shelf as well as the most productive (per unit area) in the Antarctic. High-temporal resolution profiles of acoustic backscattering strength collected from a bottom-moored, upward looking Acoustic Doppler Current Profiler were examined to describe the temporal variation of sound scattering layers. Our observations show that sound scattering layers exhibited clear diel, seasonal, and interannual pattern associated with solar radiation, sea ice concentration, and phytoplankton biomass. The timing of seasonal variation is also closely related with climate index for Southern Annular Mode and El Niño Southern Oscillation. This observation points out that acoustic signals from sound scattering layers could be a proxy to understand zooplankton ecosystem response to climate shifts.

Factors affecting Fe and Zn contents of mesozooplankton from the Costa Rica Dome.

Mineral limitation of mesozooplankton production is possible in waters with low trace metal availability. As a step toward estimating mesozooplankton Fe and Zn requirements under such conditions, we measured tissue concentrations of major and trace nutrient elements within size-fractioned zooplankton samples collected in and around the Costa Rica Upwelling Dome, a region where phytoplankton growth may be co-limited by Zn and Fe. The geometric mean C, N, P contents were 27, 5.6 and 0.21 mmol gdw-1, respectively. The values for Fe and Zn were 1230 and 498 nmol gdw-1, respectively, which are low compared with previous measurements. Migrant zooplankton caused C and P contents of the 2-5 mm fraction to increase at night relative to the day while the Fe and Zn contents decreased. Fe content increased with size while Zn content decreased with size. Fe content was strongly correlated to concentrations of two lithogenic tracers, Al and Ti. We estimate minimum Fe:C ratios in large migrant and resident mixed layer zooplankton to be 15 and 60 µmol mol-1, respectively. The ratio of Zn:C ranged from 11 µmol mol-1 for the 0.2-0.5 mm size fraction to 33 µmol mol-1 for the 2-5 mm size fraction.

A critical time window for organismal interactions in a pelagic ecosystem.

To measure organismal coherence in a pelagic ecosystem, we used moored sensors to describe the vertical dynamics of each step in the food chain in shelf waters off the west shore of Oahu, Hawaii. Horizontally extensive, intense aggregations of phytoplankton, zooplankton, and micronekton exhibited strong diel patterns in abundance and vertical distribution, resulting in a highly variable potential for interaction amongst trophic levels. Only around dusk did zooplankton layers overlap with phytoplankton layers. Shortly after sunset, micronekton ascended from the deep, aggregating on the island's shelf. Short-lived departures in migration patterns were detected in depth, vertical distribution, density, and total abundance of micronekton when zooplankton layers were present with typical patterns resuming within one hour. Layers of zooplankton began to disappear within 20 minutes of the arrival of micronekton with no layers present after 50 minutes. The effects of zooplankton layers cascaded even further up the food chain, affecting many behaviors of dolphins observed at dusk including their depth, group size, and inter-individual spacing. As a result of these changes in behavior, during a 30-minute window just after dusk, the number of feeding events observed for each dolphin and consequently the feeding time for each individual more than doubled when zooplankton layers were present. Dusk is a critical period for interactions amongst species in this system from phytoplankton to top predators. Our observations that short time windows can drive the structure and function of a complex suite of organisms highlight the importance of explicitly adding a temporal dimension at a scale relevant to individual organisms to our descriptions of heterogeneity in ocean ecosystems.

A video-plankton and microstructure profiler for the exploration of in situ connections between zooplankton and turbulence

Turbulence and mixing are ubiquitous in the environment of planktonic organisms and critical in large-scale physical and chemical oceanographic budgets. Recent studies have shown conflicting results on the contribution of zooplankton to ocean mixing as well as the impact of turbulence on zooplankton feeding and swimming behavior. Some of the confusion arises from the lack of properly resolved, simultaneous and co-located observations of zooplankton and turbulence. This paper introduces and discusses results from preliminary deployments of a Video Plankton Recorder–Vertical Microstructure Profiler (VPR–VMP), which is shown to provide good quality, high-resolution, simultaneous observations of zooplankton and turbulence. Non-turbulent shear spectra associated with a zooplankton layer are discussed in terms of automating the rejection of unreliable dissipation estimates, the shear created by individual organisms and profiler avoidance. Comparing these fine-scale observations of zooplankton and turbulence with bulk measures and possible future improvements are also discussed.

Across-shore variability in plankton layering and abundance associated with physical forcing in Monterey Bay, California

The purpose of this study was to further our understanding of the role of the coastal physical oceanographic environment as a dynamic and constantly evolving habitat for plankton. Over a 3-week period in the summer of 2010, an array of moorings were deployed and shipboard and autonomous underwater vehicle (AUV) surveys were conducted to investigate the association between physical processes and plankton distributions over the Monterey Bay, California inner shelf. Acoustic backscatter, chlorophyll-a fluorescence, and high-resolution zooplankton imagery data collected during the shipboard surveys were used to map the distributions of phytoplankton and zooplankton; and profiles of temperature, salinity, oxygen, and nitrate from the AUV characterized the physical and chemical environment. A synthesis of underway and moored time series data provided insight into the histories of water masses in the area, and facilitated tracking of internal wave groups as they propagated towards shore. A near-bottom intrusion of recently-upwelled water was found to be strongly influenced by the diurnal tide, resulting in daily across-shelf excursions past the mooring array at the 20-m isobath. Behind the leading edge of the intrusion, the water column was highly stratified in temperature, salinity, oxygen, and nitrate; and thin layers of phytoplankton and zooplankton persisted at the upper boundary of the intrusion. In ambient waters shoreward of the intrusion, stratification was weak; copepod, appendicularian, and gelatinous zooplankton abundances were relatively low; and phytoplankton and acoustic backscatter were broadly distributed throughout the lower half of the water column. The arrival of two shoreward-propagating internal wave groups observed during the shipboard survey corresponded with disparate responses in plankton distribution. In the wake of the first wave group, phytoplankton and zooplankton layers thinned or converged; in the wake of the second wave group, an eight-fold increase in gelatinous zooplankton was observed. These findings reveal a strong coupling between physical processes and the abundance and vertical distribution of plankton. It is necessary to understand the underlying physical environment in order to understand the complex distribution of organisms in the sea.

Distribution and diel vertical movements of mesopelagic scattering layers in the Red Sea

The mesopelagic zone of the Red Sea represents an extreme environment due to low food concentrations, high temperatures and low oxygen waters. Nevertheless, a 38 kHz echosounder identified at least four distinct scattering layers during the daytime, of which the 2 deepest layers resided entirely within the mesopelagic zone. Two of the acoustic layers were found above a mesopelagic oxygen minimum zone (OMZ), one layer overlapped with the OMZ, and one layer was found below the OMZ. Almost all organisms in the deep layers migrated to the near-surface waters during the night. Backscatter from a 300 kHz lowered Acoustic Doppler Current Profiler indicated a layer of zooplankton within the OMZ. They carried out DVM, yet a portion remained at mesopelagic depths during the night. Our acoustic measurements showed that the bulk of the acoustic backscatter was restricted to waters shallower than 800 m, suggesting that most of the biomass in the Red Sea resides above this depth.

Elemental composition, total lipid content, and lipid class proportions in zooplankton from the benthic boundary layer of the Beaufort Sea shelf (Canadian Arctic)

Biochemical analyses such as lipid class and elemental composition can inform us about a species’ role in community energetics and nutrient cycling. The accumulation of lipid-rich energy stores affects the elemental composition and stoichiometry of animal tissues, and this relationship is especially relevant to zooplankton at higher latitudes due to increased seasonal lipid storage. However, due to sampling difficulties, the elemental composition and energy storage capabilities of polar, benthic boundary layer zooplankton are poorly known. We determined elemental and lipid class compositions for 26 taxa of benthic boundary layer zooplankton from the Beaufort Sea shelf. Elemental composition as a percentage of dry weight ranged 21–56% for carbon (C), 4–11% for nitrogen (N), and 0.1–1.1% for phosphorus (P) across all taxa. C concentration and C:N were positively correlated with the storage lipids triacylglycerols (TG) and wax esters/steryl esters (WE/SE) and negatively correlated with membrane lipids (phospholipids and sterols). Most taxa had high levels of storage lipids, generally TG. High levels of WE/SE were found in the copepod Calanus hyperboreus (>90% of total lipid) and the chaetognath Eukrohnia hamata (72%). In contrast, the chaetognath Parasagitta elegans had only minor proportions of both TG and WE/SE. The high levels of storage lipids in most taxa indicate that feeding behavior of benthic boundary layer zooplankton on the Beaufort Sea shelf is tightly linked with seasonal pulses of epipelagic production. This is the first report on the biochemical composition of most of the amphipod and mysid taxa presented here.

Interspecific competition for food resources between Dolly Varden (Salvelinus malma) and rainbow trout (Oncorhynchus mykiss) in the Satsunai River reservoir, upper Tokachi River System, Hokkaido

To clarify interspecific competition between Dolly Varden and rainbow trout, their habitat use and stomach contents were investigated in the Satsunai River Reservoir, upper Tokachi River System, Hokkaido, Japan, during 1998-2004. During low population-densities, both species were distributed in the surface layer, whereas at high densities, Dolly Varden were restricted to the bottom layer. When terrestrial insects were abundant on the surface, they were utilized by both species. When the former were less abundant, Dolly Varden fed mainly on bottom layer zooplankton and benthic animals, indicating a shift to specific feeding niches under conditions of high fish

Observed and modeled bio-optical, bioluminescent, and physical properties during a coastal upwelling event in Monterey Bay, California

[1] During spring and summer time, coastal upwelling influences circulation and ecosystem dynamics of the Monterey Bay, California, which is recognized as a National Marine Sanctuary. Observations of physical, bio-optical properties (including bioluminescence) together with results from dynamical biochemical and bioluminescence models are used to interpret the development of the upwelling event during August 2003 in Monterey Bay, California. Observations and the biochemical model show the development of a phytoplankton bloom in the southern portion of Monterey Bay. Model results show an increase of nutrients in the southern portion of the bay, where nutrient-rich water masses are brought in by the southward flow and cyclonic circulation inside the bay. This increase in nutrients together with the sluggish circulation in the southern portion of the bay provides favorable conditions for phytoplankton growth. Our observations and models suggest that with the development of upwelling the offshore water masses with the subsurface layer of bioluminescent zooplankton were replaced by water masses advected from the northern coast of the bay with a relatively high presence of mostly nonbioluminescent phytoplankton. Inshore observations from autonomous underwater vehicles (AUVs) show consistent coincidence of chlorophyll, backscatter, and bioluminescence maxima during upwelling development. Offshore AUV observations (taken at the entrance to the bay) show a deeper bioluminescence maximum below the surface layers of high chlorophyll and backscatter values during the earlier stages of upwelling development. Later, the observed deep offshore bioluminescence maximum disappeared and became a shallower and much weaker signal, coinciding with high chlorophyll and backscatter values offshore. Based on the biochemical and bioluminescence models, a methodology for estimating the nighttime water-leaving radiance due to stimulated bioluminescence is demonstrated and evaluated.

Effects of physical structure and processes on thin zooplankton layers in Mamala Bay, Hawaii

Vertically thin layers of zooplankton were found to be common and recurring features in Mamala Bay on the south shore of the island of Oahu, Hawaii. The formation, maintenance, and vertical displacement of these thin layers are, in part, a function of regional physical oceanographic processes. The purpose of this study was to quantify general thin zooplankton layer characteristics in Mamala Bay and the underlying physical environment in which they occurred. We utilized a 2 mo time series of acoustic backscatter measurements from a calibrated acoustic Doppler current profiler (ADCP) to identify thin scattering features; took biological samples; and collected vertical profiles of physical, optical, and biological characteristics of the water column. In general, thin zooplankton lay- ers were associated with increased water column stability. Stratification at the study site was low rel- ative to other coastal regions around the continental USA where thin layers have been observed. Instances of significant stratification were short-lived, and possibly as a result, thin layers were shorter in duration. Diurnal surface heating accounted for much of the observed stratification, and the breakdown of stratification during civil twilight corresponded with a decrease in thin zooplankton layer formation. Biological and optical measurements taken during a focused shipboard profiling experiment over the course of the study suggested a mechanism for zooplankton layer formation in which zooplankton converged to graze on a thin phytoplankton layer.

Dynamic 3-dimensional structure of thin zooplankton layers is impacted by foraging fish

The potential attraction of fish to thin layers of zooplankton and the role of predation by fish in the formation and persistence of these layers were assessed using 3-dimensional sonar obser- vations. Zooplankton were found in intense layers with vertical scales of 0.2 to 4.6 m with a mode of 2.2 m. These thin zooplankton layers had complex 3-dimensional structure with significant, though gradual, undulations in their depth, thickness, and intensity. Fish spent significantly more time within zooplankton layers than expected, modifying their usual surface-coupled behavior when layers were present. Sonar tracks of individual fish showed them diving down through a zooplankton layer before spiraling slowly upwards through the layer. The upward portion of this behavior was correlated with a dramatic decrease in the intensity of zooplankton scattering at the scale of 1 m 2 , resulting in the appearance of holes in the layer. Continued observation of layers revealed that these holes slowly filled in with zooplankton an average of 4.3 min after the fish's departure. Survey results show that when more fish were observed, more holes were observed, and when larger fish were observed, larger holes were observed, so that a total of up to 5% of a layer's area could be comprised of holes. The thickness of layers was not affected by fish presence. Fish were attracted to zooplankton thin lay- ers, showing that thin layers in natural systems can have significant ecological effects; however, despite fish-associated changes to the structure of layers, the layers were resilient over time to the apparent foraging fish.