Seasons in the Ocean

How are ocean and terrestrial seasons different?

Humans are land-loving creatures and we intuitively understand that seasons are times of change; these transitions are often signaled by cues like day length or temperature. Organisms use seasonal cues like these to time events in their life cycle that maximize survival, growth, and successful reproduction. The timing of life cycle events to coincide with seasonal environmental factors is called phenology. For example, many insects, birds, and small mammals reproduce in the spring to maximize the number of warm months to grow and accumulate resources before a harsh, cold winter. We also see several examples of insect species that have co-evolved with plant species, such that insect larvae are born in synchrony with plant prey species is present or blooming.

In the ocean, we see several similar phenological patterns though the seasonal cues can be different. Most marine organisms have complex life cycles, where nearshore, bottom-associated adults spawn very small, dispersive larvae that feed in the water column for days to months before returning to the nearshore to settle, grow, and reproduce. Many fish and invertebrate species time their reproductive events such that their larvae are feeding in prey-rich conditions. Marine larvae are small organisms, often less than 10mm in length, and so they feed on very small prey items, such as phytoplankton and zooplankton (microscopic drifting plant and animal organisms). Similar to plants on land, many phytoplankton species bloom in the springtime, when day length increases and sunlight penetrates deep into the ocean. Zooplankton populations grow when there is an abundance of the phytoplankton they feed on, and fish and invertebrate larvae populations boom in response. Sunlight and food web cues play a large role in determining the phenology of marine organisms.

It is also necessary to consider that marine organisms exist in a dynamic fluid environment. Wind, currents, tide, and coastline features are all important in determining patterns of water movement in any given area. Because marine larvae are very small, they often travel or disperse with the dominant pattern of regional water movement. Many fish species are thought to time their reproduction not only according to sun-related cues, but also to water movement-related cues such that their larvae are retained in areas with favorable habitat (e.g. a nearshore rocky reef or kelp bed). Additionally, ocean conditions change dramatically along the coastline. For example, prevailing wind patterns in northern California are drastically different from wind patterns in central and northern Oregon, resulting in an abrupt change in nutrient input, phytoplankton productivity, and water movement patterns – all cues that marine organisms are tuned in to! Understanding phenology in the ocean requires an understanding of seasonality in the food web, seasonality in water movement patterns, and how these patterns are variable in space.

My dissertation research focuses on phenology of marine fish life cycles.

A major part of my dissertation research focuses on better understanding the phenology of marine fish life cycles. I study the most vulnerable life stages of the fish life cycle (larvae and juveniles, which are known to experience very high mortality rates) to better understand how food web interactions and water circulation patterns are different along the Oregon coast and throughout the year. My ultimate goal is to use the information I gain from my research to improve predictions about the future status of fish populations, especially as ocean conditions are changing.  

As a student of the ocean, my schedule aligns with ocean seasons.

As I reflect on the importance of ocean seasons on the lives of marine fishes, my study organisms, I realize that as a student of marine science I also depend on ocean seasons. In Oregon, the April-September months are marked by a transition to nutrient and phytoplankton rich waters (known as upwelling) and many larval and juvenile fish species are present in the nearshore waters. After September, the ocean transitions in to a relatively nutrient and phytoplankton poor state (known as downwelling) that lasts until the following April. The winter months also have larger storms, night tides, and less sunlight, making field work and sampling more difficult. These seasons are reflected in my work student as well: from April-September, I spend most of my time collecting samples, going on oceanographic research cruises, and organizing a team of dedicated undergraduates to conduct field work. By the time October comes around, most of my activities are land and office-based. This is a welcome change of pace after a busy summer season. This fall quarter was a special time for me because I passed my oral exams! This marks the completion of most of my coursework and the last major checkpoint in my program before my dissertation defense. As we transition into the winter term, I’m very excited to present other parts of my dissertation work to the Oregon Department of Fish and Wildlife Marine Reserves Program, and at an international conference in Japan. I’m also looking forward to teaching an introductory biology lab course, expanding my outreach opportunities, and preparing for another summer of field work!

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About Megan Wilson

Pronouns: she/her/hers. PhD Candidate in the Integrative Biology Department at Oregon State University studying marine fish population dynamics, larval ecology, and social ecological systems. Committed to STEM equity and a culturally responsive future in higher education.

2 thoughts on “Seasons in the Ocean

  1. Great post, Megan! I really like how you explained phenology and how complex it can be in the ocean. I am excited to learn more as your dissertation evolves and as changes continue to take shape along the coast. Also, a sincere congrats on passing your oral exams!! Can’t wait to hear more about your upcoming presentations here and abroad.

  2. Cycles in the ocean can certainly dictate the life of any marine scientist. Are there particular challenges for graduate students to consider when adopting this ‘cyclical’ lifestyle?

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