skip to main content

Title: Zooplankton Community Response to Seasonal Hypoxia: A Test of Three Hypotheses
Several hypotheses of how zooplankton communities respond to coastal hypoxia have been put forward in the literature over the past few decades. We explored three of those that are focused on how zooplankton composition or biomass is affected by seasonal hypoxia using data collected over two summers in Hood Canal, a seasonally-hypoxic sub-basin of Puget Sound, Washington. We conducted hydrographic profiles and zooplankton net tows at four stations, from a region in the south that annually experiences moderate hypoxia to a region in the north where oxygen remains above hypoxic levels. The specific hypotheses tested were that low oxygen leads to: (1) increased dominance of gelatinous relative to crustacean zooplankton, (2) increased dominance of cyclopoid copepods relative to calanoid copepods, and (3) overall decreased zooplankton abundance and biomass at hypoxic sites compared to where oxygen levels are high. Additionally, we examined whether the temporal stability of community structure was decreased by hypoxia. We found evidence of a shift toward more gelatinous zooplankton and lower total zooplankton abundance and biomass at hypoxic sites, but no clear increase in the dominance of cyclopoid relative to calanoid copepods. We also found the lowest variance in community structure at the most hypoxic site, in more » contrast to our prediction. Hypoxia can fundamentally alter marine ecosystems, but the impacts differ among systems. « less
Authors:
; ;
Award ID(s):
1657992 1154648
Publication Date:
NSF-PAR ID:
10169116
Journal Name:
Diversity
Volume:
12
Issue:
1
Page Range or eLocation-ID:
21
ISSN:
1424-2818
Sponsoring Org:
National Science Foundation
More Like this
  1. Increasing deoxygenation (loss of oxygen) of the ocean, including expansion of oxygen minimum zones (OMZs), is a potentially important consequence of global warming. We examined present-day variability of vertical distributions of 23 calanoid copepod species in the Eastern Tropical North Pacific (ETNP) living in locations with different water column oxygen profiles and OMZ intensity (lowest oxygen concentration and its vertical extent in a profile). Copepods and hydrographic data were collected in vertically stratified day and night MOCNESS (Multiple Opening/Closing Net and Environmental Sensing System) tows (0–1000 m) during four cruises over a decade (2007– 2017) that sampled four ETNP locations: Costa Rica Dome, Tehuantepec Bowl, and two oceanic sites further north (21– 22 N) off Mexico. The sites had different vertical oxygen profiles: some with a shallow mixed layer, abrupt thermocline, and extensive very low oxygen OMZ core; and others with a more gradual vertical development of the OMZ (broad mixed layer and upper oxycline zone) and a less extensive OMZ core where oxygen was not as low. Calanoid copepod species (including examples from the genera Eucalanus, Pleuromamma, and Lucicutia) demonstrated different distributional strategies (implying different physiological characteristics) associated with this variability. We identified sets of species that (1) changedmore »their vertical distributions and depth of maximum abundance associated with the depth and intensity of the OMZ and its oxycline inflection points; (2) shifted their depth of diapause; (3) adjusted their diel vertical migration, especially the nighttime upper depth; or (4) expanded or contracted their depth range within the mixed layer and upper part of the thermocline in association with the thickness of the aerobic epipelagic zone (habitat compression concept). These distribution depths changed by tens to hundreds of meters depending on the species, oxygen profile, and phenomenon. For example, at the lower oxycline, the depth of maximum abundance for Lucicutia hulsemannae shifted from  600 to  800 m, and the depth of diapause for Eucalanus inermis shifted from  500 to  775 m, in an expanded OMZ compared to a thinner OMZ, but remained at similar low oxygen levels in both situations. These species or life stages are examples of “hypoxiphilic” taxa. For the migrating copepod Pleuromamma abdominalis, its nighttime depth was shallow ( 20 m) when the aerobic mixed layer was thin and the low-oxygen OMZ broad, but it was much deeper ( 100 m) when the mixed layer and higher oxygen extended deeper; daytime depth in both situations was  300 m. Because temperature decreased with depth, these distributional depth shifts had metabolic implications. The upper ocean to mesopelagic depth range encompasses a complex interwoven ecosystem characterized by intricate relationships among its inhabitants and their environment. It is a critically important zone for oceanic biogeochemical and export processes and hosts key food web components for commercial fisheries. Among the zooplankton, there will likely be winners and losers with increasing ocean deoxygenation as species cope with environmental change. Changes in individual copepod species abundances, vertical distributions, and life history strategies may create potential perturbations to these intricate food webs and processes. Present-day variability provides a window into future scenarios and potential effects of deoxygenation.« less
  2. In freshwater lakes and reservoirs, climate change and eutrophication are increasing the occurrence of low-dissolved oxygen concentrations (hypoxia), which has the potential to alter the variability of zooplankton seasonal dynamics. We sampled zooplankton and physical, chemical and biological variables (e.g., temperature, dissolved oxygen, and chlorophyll a) in four reservoirs during the summer stratified period for three consecutive years. The hypolimnion (bottom waters) of two reservoirs remained oxic throughout the entire stratified period, whereas the hypolimnion of the other two reservoirs became hypoxic during the stratified period. Biomass variability (measured as the coefficient of the variation of zooplankton biomass) and compositional variability (measured as the community composition of zooplankton) of crustacean zooplankton communities were similar throughout the summer in the oxic reservoirs; however, biomass variability and compositional variability significantly increased after the onset of hypoxia in the two seasonally-hypoxic reservoirs. The increase in biomass variability in the seasonally-hypoxic reservoirs was driven largely by an increase in the variability of copepod biomass, while the increase in compositional variability was driven by increased variability in the dominance (proportion of total crustacean zooplankton biomass) of copepod taxa. Our results suggest that hypoxia may increase the seasonal variability of crustacean zooplankton communities.
  3. Abstract. Diel vertical migration (DVM) can enhance the verticalflux of carbon (C), and so contributes to the functioning of the biologicalpump in the ocean. The magnitude and efficiency of this active transport ofC may depend on the size and taxonomic structure of the migrant zooplankton.However, the impact that a variable community structure can have onzooplankton-mediated downward C flux has not been properly addressed. Thistaxonomic effect may become critically important in highly productiveeastern boundary upwelling systems (EBUSs), where high levels of zooplanktonbiomass are found in the coastal zone and are composed by a diverse communitywith variable DVM behavior. In these systems, presence of a subsurfaceoxygen minimum zone (OMZ) can impose an additional constraint to verticalmigration and so influence the downward C export. Here, we address theseissues based on a vertically stratified zooplankton sampling at threestations off northern Chile (20–30∘ S) duringNovember–December 2015. Automated analysis of zooplankton composition andtaxa-structured biomass allowed us to estimate daily migrant biomass by taxaand their amplitude of migration. We found that a higher biomass aggregatesabove the oxycline, associated with more oxygenated surface waters and thiswas more evident upon a more intense OMZ. Some taxonomic groups, however,were found closely associated with the OMZ. Most taxa were able to performDVM in the upwellingmore »zone withstanding severe hypoxia. Also, strongmigrants, such as eucalanid copepods and euphausiids, can exhibit a largemigration amplitude (∼500 m), remaining either temporarily orpermanently within the core of the OMZ and thus contributing to the releaseof C below the thermocline. Our estimates of DVM-mediated C flux suggestedthat a mean migrant biomass of ca. 958 mg C m−2 d−1 may contributewith about 71.3 mg C m−2 d−1 to the OMZ system through respiration,mortality and C excretion at depth, accounting for ca. 4 % of the netprimary production, and so implies the existence of an efficient mechanismto incorporate freshly produced C into the OMZ. This downward C fluxmediated by zooplankton is however spatially variable and mostly dependenton the taxonomic structure due to variable migration amplitude and DVMbehavior.« less
  4. Observing multiple size classes of organisms, along with oceanographic properties and water mass origins, can improve our understanding of the drivers of aggregations, yet acquiring these measurements remains a fundamental challenge in biological oceanography. By deploying multiple biological sampling systems, from conventional bottle and net sampling to in situ imaging and acoustics, we describe the spatial patterns of different size classes of marine organisms (several microns to ∼10 cm) in relation to local and regional (m to km) physical oceanographic conditions on the Delaware continental shelf. The imaging and acoustic systems deployed included (in ascending order of target organism size) an imaging flow cytometer (CytoSense), a digital holographic imaging system (HOLOCAM), an In Situ Ichthyoplankton Imaging System (ISIIS, 2 cameras with different pixel resolutions), and multi-frequency acoustics (SIMRAD, 18 and 38 kHz). Spatial patterns generated by the different systems showed size-dependent aggregations and differing connections to horizontal and vertical salinity and temperature gradients that would not have been detected with traditional station-based sampling (∼9-km resolution). A direct comparison of the two ISIIS cameras showed composition and spatial patchiness changes that depended on the organism size, morphology, and camera pixel resolution. Large zooplankton near the surface, primarily composed of appendicularians andmore »gelatinous organisms, tended to be more abundant offshore near the shelf break. This region was also associated with high phytoplankton biomass and higher overall organism abundances in the ISIIS, acoustics, and targeted net sampling. In contrast, the inshore region was dominated by hard-bodied zooplankton and had relatively low acoustic backscatter. The nets showed a community dominated by copepods, but they also showed high relative abundances of soft-bodied organisms in the offshore region where these organisms were quantified by the ISIIS. The HOLOCAM detected dense patches of ciliates that were too small to be captured in the nets or ISIIS imagery. This near-simultaneous deployment of different systems enables the description of the spatial patterns of different organism size classes, their spatial relation to potential prey and predators, and their association with specific oceanographic conditions. These datasets can also be used to evaluate the efficacy of sampling techniques, ultimately aiding in the design of efficient, hypothesis-driven sampling programs that incorporate these complementary technologies.« less
  5. Effects of climate change-driven disturbance on lake ecosystems can be subtle; indirect effects include increased nutrient loading that could impact ecosystem function. We designed a low-level fertilization experiment to mimic persistent, climate change-driven disturbances (deeper thaw, greater weathering, or thermokarst failure) delivering nutrients to arctic lakes. We measured responses of pelagic trophic levels over 12 yr in a fertilized deep lake with fish and a shallow fishless lake, compared to paired reference lakes, and monitored recovery for 6 yr. Relative to prefertilization in the deep lake, we observed a maximum pelagic response in chl a (+201%), dissolved oxygen (DO, _43%), and zooplankton biomass (+88%) during the fertilization period (2001–2012). Other responses to fertilization, such as water transparency and fish relative abundance, were delayed, but both ultimately declined. Phyto- and zooplankton biomass and community composition shifted with fertilization. The effects of fertilization were less pronounced in the paired shallow lakes, because of a natural thermokarst failure likely impacting the reference lake. In the deep lake there was (a) moderate resistance to change in ecosystem functions at all trophic levels, (b) eventual responses were often nonlinear, and (c) postfertilization recovery (return) times were most rapid at the base of the food webmore »(2–4 yr) while higher trophic levels failed to recover after 6 yr. The timing and magnitude of responses to fertilization in these arctic lakes were similar to responses in other lakes, suggesting indirect effects of climate change that modify nutrient inputs may affect many lakes in the future.« less