Liam Hubbert

Topic

Exploring spatiotemporal variability in secondary production off the west coast of Vancouver Island using biochemical approaches

School of Earth and Ocean Sciences

Date & location

• Wednesday, January 24, 2024
• 1:00 P.M.
• Clearihue Building, Room B007

Examining Committee

Supervisory Committee

• Dr. John Dower, School of Earth and Ocean Sciences, University of Victoria (Co-Supervisor)
• Dr. Akash Sastri, Department of Biology, UVic (Co-Supervisor)
• Dr. Catherine Stevens, School of Earth and Ocean Sciences, UVic (Member)

External Examiner

• Dr. David Kimmel, Alaska Fisheries Science Center, National Oceanic and Atmospheric Administration

Chair of Oral Examination

• Dr. Paul Schure, Department of Economics, UVic

Abstract

Secondary production in marine ecosystems refers to the rate at which zooplankton biomass increases through a combination of somatic and reproductive growth. Despite its importance in understanding the flow of energy to higher trophic levels, in situ measurements of secondary production rates in marine ecosystems remain rare. In recent decades, biochemical methods of estimating secondary production have become increasingly popular, though there still exist critical knowledge gaps as to how effective these methods are at estimating in situ growth and production rates. Addressing these knowledge gaps is necessary to lay the foundation for the future integration of routine secondary production rate measurements as part of synoptic oceanographic surveys.

Chapter 1 of this thesis introduces the global importance of zooplankton and reviews the methods currently used to assess zooplankton production rates. Specifically, two contemporary biochemical methods are discussed, as well as their advantages and limitations as compared to more traditional incubation methods. The first is the aminoacyl-TRNA synthetases (AARS) method, where the activity of AARS enzymes is utilized to derive a proxy measure of growth rate. The second is the chitobiase method, in which the rate of decay of the free chitobiase enzyme in water is used to estimate the growth and production rates of crustacean zooplankton assemblages. The chapter concludes with a description of the regional oceanographic setting in which these studies took place and outlines the primary objectives of this thesis.

Chapter 2 focuses on the AARS method of measuring secondary production rates. Here, the efficacy of this method for mixed zooplankton assemblages was assessed by comparing growth and production rate estimates to those predicted from two widely used empirical models. Samples collected from eight stations off the West Coast of Vancouver Island (WCVI) in September 2021 were used to measure total AARS and protein-specific AARS (spAARS) activities. Total AARS showed strong positive correlations with production rates predicted by both models, whereas correlations with spAARS were weaker. Spatial variation in AARS activity showed that higher production rates were observed in the inshore regions of the study area, and lower rates were observed offshore. These results indicated that in situ AARS-based production rates are temperature-dependent and show significant variation with total zooplankton biomass.

In Chapter 3, the chitobiase method of estimating secondary production rate was used to assess production rates in the waters off the WCVI in September 2022 and May 2023. Water samples were collected from the four distinct bioregions off the WCVI during each sampling period, along with zooplankton net samples for biomass and taxonomy analyses. The data gathered from these samples were used to better understand how production rates vary between regions with distinct oceanographic characteristics. Chitobiase biomass-production rate (BPR) and growth rate estimates (daily production to biomass ratio) varied with both season and region, though the trends in these rates did not align with trends in mixed-layer temperature and biomass. Higher chitobiase-based growth rates were observed in inshore regions during September 2022 and May 2023. Chitobiase BPR in September 2022 also followed this trend. Conversely, production rates in May 2023 were higher in the south, indicating a change in the regional drivers of production between seasons. The chitobiase-based growth and production rate estimates obtained during this study were also added to the growing time series of previous chitobiase measurements in this region and indicate that production rates have recovered since the low values measured following the 2014-2016 marine heatwave.

Chapter 4 of this thesis presents general conclusions on how the AARS method can be used in future studies, as well as the ecological and methodological challenges faced during this study. This thesis concludes with suggestions of how these methods can be utilized in the future to gain a greater understanding of in situ zooplankton community production rates.