Fine-scale vertical distribution of zooplankton in the oxygen minimum zone off Peru.

Palash, Sahed Ahmed (2019) Fine-scale vertical distribution of zooplankton in the oxygen minimum zone off Peru. Open Access (Master thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 57 pp.

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Abstract

In the modern ocean, waters with <20 μmol kg-¹ of dissolved oxygen are known ⁻ as oxygen minimum zones (OMZs) and occupy ~1% of the global ocean. Zooplankton plays an important role in marine ecosystems, as a trophic link between primary producers and larger animals. Zooplankton is also an integral part of the biological carbon pump. Diel vertical migrations (DVM) between midwater depths at daytime and surface waters at nighttime is a unique process conducted by zooplankton in the ocean that impacts ecological
interactions and nutrients cycles. Zooplankton DVM can be responsible for the transfer of a significant amount of carbon and nutrient from the surface to depth. Most available data on the vertical distribution of zooplankton rely on vertically stratified net samples with relatively thick integrated depth strata sampled.
Above the intense OMZ off Peru, the upper oxycline is viewed as the single most important barrier structuring the water column. Up to now, the fine-scale distribution of zooplankton relative to this barrier has not been resolved in detail. In the present study, I determined the fine-scale vertical distribution of zooplankton abundance and biomass and its correlation to O2. During the cruise, zooplankton was collected at eight stations in different O2 strata from the upper OMZ to the surface using a towed Hydrobios multinet,
with three nets sampling the oxycline (which was often only a few meters thick. At the same stations, vertical hauls with fixed depth intervals (1000-600-300-200-100-0) to a maximum of 1000 m depth were conducted.
Zooplankton >500 μm from the trawl hauls and >1000 μm from the vertical hauls were used for the data analysis. The oxycline (Zoxy) and thermocline (Ztherm) in the OMZ off Peru ranged between ~20-50 m and were shallower at onshore compared to offshore stations indication of higher biomass distribution but similar abundance. In terms of abundance, calanoida and eucalanidae copepods are the most dominant organisms in the system. However, both in terms of biomass and in migrating biomass, euphausiacea (in waters deeper than 250m) and the endemic P. monodon (on the shelf) are the most important contributors to nighttime biomass in surface waters. Differences in zooplankton day/night abundances and biomass in different oxygen bins indicate DVM from the surface to OMZ and vice-versa with the complete absence of euphausiacea and P. monodon in the surface water during the day. During DVM, calanoida and eucalanidae show accumulation in the water column just on top of the oxycline where dissolved oxygen concentration ranges from 0-20
μmol kg⁻¹ during the day. During the night, P. monodon along with calanoida and eucalanidae show accumulation in the more oxygenated near-surface bins. The biomass distribution during the night and day in different O2 bins have similar trends of having large size copepods down towards oxycline during the day and other-way-around at night (ANOVA, p < 0.017 for calanoida, p < 0.01 for eucalanidae). The nighttime distribution of euphausiacea is not restricted to the oxygenated surface water (ANOVA, p < 0.00000001) but reached into hypoxic conditions in the oxycline, indicating that the oxycline may be a hot spot of bacterial activity where anoxic metabolic pathways are periodically supplied with zooplankton excretion and defecation products. Variability in catch efficiency and zooplankton distribution between vertical and trawl sampling is providing us an opportunity to determine the best technique from these two. Last but not the least, a study needs to carry for the increasing taxonomic resolution along with fine-tuning of the imaging instrument (capturing 4D zooplankton community dynamics) to determine the exact species and their depth
preferences in terms of oxygen concentrations.

Document Type: Thesis (Master thesis)
Thesis Advisor: Kiko, Rainer and Wahl, Martin
Keywords: Eastern Tropical South Pacific (ETSP), Oxygen Minimum Zones, Zooplankton Community, Diel Vertical Migration (DVM) and Biomass Distribution
Subjects: Course of study: MSc Biological Oceanography
Research affiliation: OceanRep > GEOMAR > FB3 Marine Ecology > FB3-EOE-B Experimental Ecology - Benthic Ecology
Date Deposited: 13 Dec 2019 08:19
Last Modified: 22 Jan 2020 12:28
URI: http://oceanrep.geomar.de/id/eprint/48436

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