Diatom Community Composition Shifts Driven by Coherent Cyclonic Mesoscale Eddies in the California Current System

The California Current System (CCS) is characterized by an equatorward flowing eastern boundary current, as well as seasonal wind-driven coastal upwelling which supplies nutrient-rich waters to the surface and drives high coastal productivity. Cyclonic mesoscale eddies form off the coast in the CCS where they trap the highly productive upwelled coastal waters, along with their resident planktonic communities, and transport them offshore into the more oligotrophic California Current waters. The interaction between waters within and outside of the eddies is limited, and so the eddies act as natural mesocosms, where the resident phytoplankton population undergo ecological succession as the eddy ages. Diatoms, a unicellular and eukaryotic subgroup of phytoplankton, have high sensitivities to changes in their environment, particularly temperature and nutrient distributions. In this study, I examine how diatom communities trapped within mesoscale eddies in the CCS evolve in response to environmental shifts as they travel offshore. In a transect that bisected two cyclonic eddies off the coast of northern California near Cape Mendocino, diatom samples were collected and later sequenced using high throughput sequencing. Although the eddies both originated in broadly the same location, they had formed 2 and 10 months previous to sampling, respectively. Because of this difference in the age of the eddies, I can approximate the time evolution of a single CCS eddy by comparing their biogeochemical and ecological characteristics. The older, offshore eddy was low in macronutrients, nitrate-limited, low in Fe, and lower in diversity, the last result largely driven by the relative abundance of a single Rhizosolenia species. Rhizosolenia accounted for over 50% of the diatom community in 5 out of 8 offshore eddy stations, with one of these stations characterized by nearly 75% Rhizosolenia. Our results suggest that the Rhizosolenia species present in the offshore eddy is one that bypasses nitrate limitation by forming vertically migrating mats. I also found elevated relative abundances of Pseudo-nitzschia cf. sp. and Thalassiothrix sp. in the offshore eddy. The younger, nearshore eddy was higher in macronutrients, Fe-limited, and higher in diversity. Top abundances for this eddy include Pseudo-nitzschia sp., Fragilariopsis kerguelensis, F. cf. kerguelensis, Thalassiosira ritscheri, Asteromphalus sp., and T. oestrupii. Our results show that the biogeochemistry and diatom community structure within cyclonic eddies evolve as the eddies move offshore from the coast. The high-nutrient coastal waters are initially dominated by coastal diatoms known to have higher nutrient requirements. As the nutrients within the eddy are drawn down over time, species equipped with low-nutrient adaptations can become dominant. The combined effect of transport by, and ecological succession within the eddies is likely a key factor in mediating carbon cycling and export across the wider CCS region

Examining Ecological Succession of Diatoms in California Current System Cyclonic Mesoscale Eddies

The California Current System is a diatom-dominated region characterized by seasonal coastal upwelling and additional elevated mesoscale activity. Cyclonic mesoscale eddies in the region trap productive coastal waters with their planktonic communities and transport them offshore with limited interaction with surrounding waters, effectively acting as natural mesocosms, where phytoplankton populations undergo ecological succession as eddies age. This study examines diatom community composition within two mesoscale cyclonic eddies that formed in the same region of the California Current System 2 months apart and in the California Current waters surrounding them. The diatom communities were analyzed in the context of shifting environmental gradients and through a lens of community succession to expand our understanding of biophysical interactions in California Current System cyclonic eddies. Diatom communities within each eddy were different from non-eddy communities and varied in concert with salinity and dissolved iron (Fe) concentrations. The younger, nearshore eddy displayed higher macronutrient and dissolved Fe concentrations, had higher values for diatom Shannon diversity and evenness, and had nutrient ratios indicative of either eventual silicic acid (Si) or Fe limitation or possibly co-limitation. The older, offshore eddy displayed low macronutrient and dissolved Fe concentrations, was likely nitrate-limited, and had lower diatom Shannon diversity and evenness indices. Sequences from the genus Rhizosolenia, some of which form vertically migrating mats to bypass nitrate limitation, dominated in the older eddy. This is of potential significance as the prevalence of Rhizosolenia mats could impact estimates of carbon cycling and export in the wider California coastal area

Interactions of Bioactive Trace Metals in Shipboard Southern Ocean Incubation Experiments

In the Southern Ocean, it is well‐known that iron (Fe) limits phytoplankton growth. Yet, other trace metals can also affect phytoplankton physiology. This study investigated feedbacks between phytoplankton growth and dissolved Fe, manganese (Mn), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), and cadmium (Cd) concentrations in Southern Ocean shipboard incubations. Three experiments were conducted in September–October 2016 near the West Antarctic Peninsula: Incubations 1 and 3 offshore in the Antarctic Circumpolar Current, and Incubation 2 inshore in Bransfield Strait. Additions of Fe and/or vitamin B12 to inshore and offshore waters were employed and allowed assessment of metal (M) uptake relative to soluble reactive phosphorus (P) across a wide range of initial conditions. Offshore, treatments of \u3e1 nmol L−1 added Fe were Fe‐replete, whereas inshore waters were already Fe‐replete. Results suggest Mn was a secondary limiting nutrient inshore and offshore. No Fe‐vitamin B12 colimitation was observed. Overall, M:P uptake in the incubations was closely related to initial dissolved M:P for Fe, Mn, Co, Ni, and Cd, and for Cu inshore. Final concentrations of Fe and Zn were similar across light treatments of the experiments despite very different phytoplankton responses, and we observed evidence for Co/Cd/Zn substitution and for recycling of biogenic metals as inventories plateaued. In dark bottles, the absence of Mn oxidation may have allowed more efficient recycling of Fe and other trace metals. Our results provide insight into factors governing trace metal uptake, with implications for phytoplankton community composition locally and preformed micronutrient bioavailability in Southern Ocean water masses

Diatom Community Composition Shifts in the Nitrogen-limited Mid-Atlantic Bight

Diatoms are unicellular, photosynthetic eukaryotes with high sensitivities to nutrient availability, often leading to rapid shifts in community composition, making diatoms strong bioindicators for environmental change. The Mid-Atlantic Bight (MAB), located off the eastern U.S., is an ecosystem predominantly limited by nitrogen and is home to a complex distribution of nutrients due to coastal waters interacting with the Gulf Stream. In the summer, waters are strongly stratified and phytoplankton communities are dominated by diatoms. Rivers and estuaries in this region can deliver nitrogen pulses, which can lead to community shifts in response to nutrient availability. Previous studies have produced overall phytoplankton composition for this region, naming the top few diatoms present, though have not provided an in-depth species-level diatom community composition analysis for the MAB. We completed a comprehensive surface water diatom species level assemblage analysis along the MAB to help elucidate the coastal-driven effects on the ecosystem as a function of distance from the coast. In order to obtain the species-level resolution, high-throughput sequencing techniques were used on the V4 region of the 18S rDNA marker gene. Quantitative PCR was also performed to acquire a more accurate assessment of the quantity of diatoms present in each sample. The community compositions, when analyzed spatially, highlight the locally adapted, coastal water community being dominated by Leptocylindrus, Cylindrotheca, and Rhizosolenia. Offshore Gulf Stream waters were dominated by Fragilariopsis and Pseudo-nitzschia.https://digitalcommons.odu.edu/sciences_achievement/1000/thumbnail.jp