Most changes in the ocean happen long before we notice them.
The water still looks blue. Waves continue to break across the sandbars. Beachgoers spread their towels beneath the same summer sun, children chase ghost crabs along the tide line, and anglers cast into the surf hoping for a bite.
Yet beneath the surface, a warming ocean is altering the conditions that shape life along the coast.
The changes begin with microscopic organisms drifting through the water column and ripple outward through fish, shellfish, jellyfish, and eventually the people who swim, fish, and play in these waters. Scientists have documented rising ocean temperatures worldwide, with the ocean absorbing the vast majority of the excess heat generated by a warming climate (IPCC, 2023; NASA, 2025).
For beachgoers along the Onslow County coast, these changes often appear as small observations. Water that feels warmer than it once did. Green swirls visible in drone photographs. Jellyfish gathering along the shoreline. Questions about bacteria, shellfish closures, and changing fish patterns.
At first glance, these may seem unrelated.
In reality, they are all connected.
A Longer Summer Beneath the Surface
The ocean does not warm as quickly as the air above it, but it holds heat much longer.
As coastal waters warm earlier in spring and remain warm later into autumn, marine organisms experience something similar to a longer growing season on land. Processes that once occurred over a few summer months may now persist for much longer periods – lasting later into the year or shifting the timing of organisms that are responding to environmental conditions (IPCC, 2023; Menzel et al., 2006).
For marine life, temperature influences nearly everything. Growth rates, feeding behavior, reproduction, migration, and metabolism are all affected by the warmth of the surrounding water (Pörtner & Knust, 2007).
For many species, warmer water means increased biological activity. But every response carries consequences that ripple through the food web.
The first organisms to respond are often the smallest.
When Tiny Things Respond First
Most beachgoers never think about what is suspended in the water around them.
Unlike a forest, marsh, or coral reef, much of the ocean’s life is not immediately visible. Looking across the surf, the water may appear empty except for an occasional fish, jellyfish, or diving bird.
Yet the water column itself is home to countless drifting organisms. Some are microscopic plants. Others are microscopic animals. Together, they form a community known as plankton.
Among the most important are phytoplankton—tiny plant-like organisms that drift with currents and tides. Though nearly invisible to the naked eye, they capture sunlight, form the foundation of marine food webs, and produce much of the oxygen found in Earth’s atmosphere (Falkowski et al., 1998).
Feeding on them are zooplankton, a diverse group of drifting animals that includes tiny crustaceans, larval fish, and the early life stages of many marine organisms. Nearly everything in the ocean depends on this microscopic world in some way.
As temperatures rise and sunlight remains abundant, phytoplankton growth can increase. In many cases, this increased productivity benefits marine ecosystems by providing more food for zooplankton, shellfish, and small fish.
Sometimes, however, the changes become visible.
Drone photographs, fishing reports, and satellite imagery occasionally reveal ribbons and swirls of green water along the coastline. Many people assume these colors indicate pollution, but the explanation is often more complex.
In some cases, the green color reflects increased concentrations of phytoplankton. In others, it may result from suspended sediment, river discharge, or other naturally occurring materials in the water (Behrenfeld et al., 2006).
Green water does not automatically mean unhealthy water.
More often, it is a visible reminder that biological activity is taking place beneath the surface—activity that most beachgoers never see.
In fact, many periods of greener water reflect productive conditions that support marine food webs. Increased phytoplankton can provide more food for zooplankton, shellfish, and small fish, creating benefits that ripple through the ecosystem. The presence of abundant microscopic life is often a sign that the ocean is actively supporting the organisms that depend upon it.
Not all blooms are beneficial, however.
Occasionally, beachgoers hear news reports about harmful algal blooms and wonder whether the water they are seeing is part of one.
Under certain conditions, a small number of phytoplankton species can reproduce so rapidly that they begin affecting the ecosystem around them. These events are known as harmful algal blooms.
Along U.S. coastlines, some of the better-known examples include Karenia brevis, which causes many Gulf Coast red tides; Alexandrium species, which can produce toxins associated with paralytic shellfish poisoning; and Pseudo-nitzschia, which produces domoic acid and has been linked to shellfish closures and wildlife impacts in several regions (Anderson et al., 2012; Trainer et al., 2012).
Unlike the seasonal increases in phytoplankton that help support marine food webs, harmful blooms can stress marine life and create concerns for people. Some produce toxins that accumulate in shellfish, leading to temporary harvesting closures. Others contribute to oxygen declines as large concentrations of algae die and decompose (Anderson et al., 2002).
For beachgoers, the challenge is that harmful blooms do not always look the way people expect. A bloom may appear as a patch of unusually dense water, a streak of discoloration, or sometimes little different from surrounding water. Color alone rarely tells the whole story.
In some cases, blooms may discolor the water, turning it red, rust-colored, brown, orange, or an unusually dense green. Some may also produce odors that people describe as sulfur-like, fishy, or similar to decaying vegetation (Gilbert et al., 2005; Gobler, 2020).
Fortunately, most periods of pale green, emerald green, or slightly tea-colored water along the Carolina coast are not harmful algal blooms. More often, they reflect normal concentrations of phytoplankton, suspended sediment, river discharge, or other natural processes.
The challenge is that the water does not always reveal which is which at first glance. What appears to be a simple color change may be telling a much more complicated story beneath the surface.
The Oxygen Paradox
Warm water creates a biological contradiction.
As temperatures rise, marine organisms require more oxygen to stay active and carry out basic life processes. At the same time, warmer water naturally holds less dissolved oxygen—the tiny oxygen molecules mixed into the water that fish, crabs, and many other marine animals breathe. Unlike oxygen in the air around us, this oxygen must remain suspended within the water itself, and warmer water cannot hold as much of it as cooler water (Keeling et al., 2010).
In other words, as the demand for oxygen increases, the supply decreases. Scientists refer to this growing challenge as ocean deoxygenation, a phenomenon driven in part by warming oceans and documented in coastal waters around the world (Breitburg et al., 2018; Diaz & Rosenberg, 2008).
The effects are often invisible to beachgoers. Unlike a jellyfish bloom or a patch of green water, low oxygen leaves few obvious clues for someone standing on the shoreline.
Fish may become sluggish, gather near inlets or channels, or disappear from places where they are normally common long before any obvious signs appear at the surface. Crabs, shrimp, and other marine organisms must work harder to find places with enough oxygen to survive. Some may move into shallower water, concentrate in tidal channels, or bury themselves in sediment where conditions remain tolerable. Some areas become less favorable, while others provide temporary pockets of suitable habitat.
The ocean begins to rearrange itself.
Following the Fish
Stand on the beach long enough and patterns begin to emerge.
A stretch of water that looked empty an hour ago suddenly flickers with baitfish. Birds gather over a patch of surf. A school of fish appears just beyond a sandbar, then vanishes as quickly as it arrived.
Most of these movements happen without drawing much attention. To someone walking the shoreline, the ocean can seem unchanged from one day to the next.
Beneath the surface, however, marine life is constantly adjusting.
Fish are not fixed to one place. They move through the water searching for conditions that suit them, often responding to changes that people cannot see. A slight difference in temperature, a pocket of water with more oxygen, or a concentration of prey can be enough to shift where fish gather (Pörtner & Knust, 2007).
Along the beaches of Onslow County, these adjustments may be playing out right in front of us.
Anglers sometimes notice schools of mullet, menhaden, silversides, or other baitfish stacked along a sandbar. Predatory fish such as bluefish, Spanish mackerel, red drum, or even small sharks may linger near an inlet. Feeding activity may suddenly erupt close to shore, with baitfish leaping from the water as predators chase them, birds diving repeatedly into the surf, and flashes of silver visible just beyond the breakers. Tides, currents, and seasonal migrations all help shape these patterns, but fish are also responding to the changing conditions around them.
Even the breaking surf can matter.
Where waves tumble across shallow bars, the water is constantly being mixed and stirred. Oxygen from the atmosphere is worked back into the water, creating conditions with higher oxygen levels than nearby areas where the water is calmer and moves less. What looks like nothing more than a line of breaking waves can become a place where marine life gathers.
Most beachgoers never notice these subtle shifts.
They simply see fish where fish happen to be.
Yet changing ocean conditions are becoming an increasingly important part of the story. Fish may feed in a different stretch of surf than usual, baitfish may gather in unexpected places, or seasonal arrivals may occur a little earlier or later than expected. Most of the time, the reasons remain hidden beneath the surface, but the movements themselves reveal that marine life is responding to a changing ocean (Pinsky et al., 2013).
The ocean is not standing still.
And neither are the fish.
The Species That Thrive
Not every organism responds to warming water in the same way.
Some struggle.
Others thrive.
For many beachgoers, one of the most noticeable signs of seasonal change arrives as translucent shapes drifting through the surf. As plankton populations increase and warm conditions persist, the same environmental changes influencing fish and other marine life can also create favorable conditions for jellyfish.
Jellyfish are a familiar part of coastal life in Onslow County, but their numbers can vary dramatically from season to season. During late spring, summer, and early fall, when air and water temperatures commonly reach about 68–86°F (20–30°C), conditions often become more favorable for larger jellyfish populations than during the colder months.
Most people first notice them while wading in the shallows, scanning the water from a pier, or walking the beach after a storm. A shoreline that seemed empty a few weeks earlier may suddenly hold dozens of stranded jellyfish along the tide line. Depending on the season, visitors might encounter moon jellies pulsing just beneath the surface, cannonball jellies washing ashore in clusters, or the unmistakable blue floats of Portuguese man o’ war carried in by winds and currents.
These appearances can feel sudden, but they rarely are.
A shoreline that seems free of jellyfish one week may be dotted with them the next. To someone standing on the beach, it can feel as though they arrived overnight.
Much of a jellyfish’s life unfolds out of sight. Many drift offshore, while others pass through life stages that most people never notice. As waters warm and food becomes more abundant, conditions can support larger populations. Sometimes the result is a bloom—a period when unusually large numbers gather in coastal waters and become difficult to ignore.
In reality, the conditions that support them may have been developing for weeks or even months. While warmer water does not automatically mean more jellyfish everywhere (Condon et al., 2012), seasonal warming can contribute to periods when jellyfish become unusually abundant in nearshore waters. Currents, food availability, and other environmental factors also influence when and where these blooms occur (Purcell, 2005; Richardson et al., 2009).
For observers on the shore, jellyfish are often among the first visible reminders that changes in ocean conditions do not stay hidden beneath the surface for long.
Not every organism has the ability to drift or swim away.
The Organisms That Cannot Leave
Fish can relocate.
Jellyfish can drift with currents.
Shellfish remain where they are.
For many beachgoers, oysters, clams, and mussels are simply part of the coastal landscape—something encountered at low tide, served at a seafood restaurant, or harvested during shellfish season.
Yet these animals spend their lives doing something remarkable.
Oysters, clams, mussels, and other shellfish continuously draw water through their bodies, removing microscopic food particles as they feed. This is why they are known as filter feeders. A single adult oyster can filter up to 50 gallons (190 L) of water per day, depending on temperature, salinity, and other environmental factors (Jansen, 2023; zu Ermgassen et al., 2012).
An oyster reef does not simply sit on the bottom. Day and night, every oyster is quietly filtering the estuary around it.
Because they process so much water, shellfish become closely connected to the conditions around them. Changes in temperature, oxygen levels, harmful algal blooms, and water quality can all affect their health and survival (Shumway, 1990).
For this reason, shellfish often serve as some of the earliest indicators that environmental conditions have changed.
When shellfish harvesting areas are temporarily closed, many people assume pollution is the only explanation. In reality, closures may occur for a variety of reasons, including elevated levels of bacteria such as fecal coliforms, Escherichia coli (E. coli), or Enterococcus, harmful algal blooms, or other conditions that could affect human health (Food & Drug Administration (FDA), 2023).
In many cases, these closures are evidence that monitoring programs are working exactly as intended.
The shellfish are not causing the problem.
They are revealing it.
By filtering the surrounding water day after day, they provide a glimpse into conditions that might otherwise go unnoticed.
Sometimes, what they reveal is a bacterium that has received increasing attention in recent years.
The Bacteria That Was Already Here
Few marine organisms have generated more public concern in recent summers than Vibrio bacteria.
News headlines often make it sound like a new arrival.
It is not.
Like the phytoplankton, zooplankton, and countless other organisms drifting through coastal waters, Vibrio vulnificus has always been part of the hidden community beneath the surface.
Most beachgoers never notice it. They cannot see it. They do not think about it while wading through the surf or collecting shells along the shoreline.
Yet these bacteria have long occupied an important ecological role.
Vibrio species occur naturally in coastal and estuarine waters around the world. They help break down organic matter and recycle nutrients, returning materials to the food web where they can be used again by other organisms. If they were somehow eradicated, scientists would expect dead plants, algae, fish, and other organic material to break down more slowly. Over time, beach wrack could linger longer along shorelines, decaying material could accumulate in marshes and tidal flats, and nutrients normally returned to the water and sediment would become less available to the organisms that depend on them. These changes might not be obvious at first, but they could gradually alter the health and productivity of coastal ecosystems (Oliver, 2005).
What often changes first is not the presence of these organisms, but their abundance.
Just as warmer conditions can influence phytoplankton growth, they can also affect microbial communities.
Most of the time, these changes remain invisible.
The water may look the same. The beach may feel the same. Nothing about a morning walk along the shoreline suggests that microscopic populations are shifting beneath the surface.
Yet they are.
When water temperatures rise well above the normal seasonal range for a region and remain elevated for extended periods, conditions can become more favorable for certain Vibrio species. Their populations may increase, raising the likelihood of human exposure (Baker-Austin et al., 2012; Baker-Austin et al., 2018).
For most healthy beachgoers, swimming in coastal waters remains a normal part of enjoying the beach.
However, individuals with open wounds, compromised immune systems, or underlying health conditions may face greater risks and should pay closer attention to local advisories and public health guidance.
The story is not really about a dangerous bacterium suddenly appearing where it did not belong.
It is another example of a broader pattern that runs throughout coastal ecosystems.
As environmental conditions change, the organisms already living there respond. Some become more abundant. Others become less common. Together, their responses reveal something easy to miss while standing at the water’s edge: the shoreline is alive with countless forms of life that most of us never see.
Even the smallest inhabitants are connected to the larger changes unfolding around them.
Reading the Signs
Most beachgoers will never measure dissolved oxygen or monitor water temperatures.
What they will notice are the signs: water that stays warm later into autumn, green swirls visible from a fishing pier or drone photograph, jellyfish gathering along a tide line, fish appearing in unexpected places, temporary shellfish closures, or questions about bacteria that have long existed in coastal waters.
None of these observations tells the whole story on its own.
Taken together, however, they reveal an ecosystem responding to warmer conditions one species, one season, and one degree at a time.
Looking Beneath the Surface
Most changes in the ocean begin out of sight.
Long before beachgoers notice a jellyfish drifting through the surf or a patch of green water offshore, microscopic organisms are already responding to changing conditions. Fish adjust their movements. Oxygen levels shift. Shellfish filter whatever the water brings.
By the time we notice the signs, the ecosystem has often been responding for weeks or months.
The water may feel the same as it always has beneath our feet. Yet each summer offers new clues about the changes taking place below the surface—if we know where to look.
References
Anderson, D. M., Cembella, A. D., & Hallegraeff, G. M. (2012). Progress in understanding harmful algal blooms: Paradigm shifts and new technologies for research, monitoring, and management. Annual Review of Marine Science, 4(1), 143-176. https://doi.org/10.1146/annurev-marine-120308-081121
Anderson, D. M., Glibert, P. M., & Burkholder, J. M. (2002). Harmful algal blooms and eutrophication: Nutrient sources, composition, and consequences. Estuaries, 25(4), 704-726. https://doi.org/10.1007/bf02804901
Baker-Austin, C., Oliver, J. D., Alam, M., Ali, A., Waldor, M. K., Qadri, F., & Martinez-Urtaza, J. (2018). Vibrio spp. infections. Nature Reviews Disease Primers, 4(1), 1-9. https://www.nature.com/articles/s41572-018-0005-8
Baker-Austin, C., Trinanes, J. A., Taylor, N. G., Hartnell, R., Siitonen, A., & Martinez-Urtaza, J. (2012). Emerging vibrio risk at high latitudes in response to ocean warming. Nature Climate Change, 3(1), 73-77. https://doi.org/10.1038/nclimate1628
Behrenfeld, M. J., O’Malley, R. T., Siegel, D. A., McClain, C. R., Sarmiento, J. L., Feldman, G. C., Milligan, A. J., Falkowski, P. G., Letelier, R. M., & Boss, E. S. (2006). Climate-driven trends in contemporary ocean productivity. Nature, 444(7120), 752-755. https://doi.org/10.1038/nature05317
Breitberg, D., Levin, L. A., Oschlies, A., Grégoire, M., Chavez, F. P., Conley, D. J., Garçon, V., Gilbert, D., Gutiérrez, D., & Zhang, J. (2018). Declining oxygen in the global ocean and coastal waters. Science, 359(6371). https://doi.org/10.1126/science.aam7240
Condon, R. H., Graham, W. M., Duarte, C. M., Pitt, K. A., Lucas, C. H., Haddock, S. H., Sutherland, K. R., Robinson, K. L., Dawson, M. N., Decker, M. B., Mills, C. E., Purcell, J. E., Malej, A., Mianzan, H., Uye, S., Gelcich, S., & Madin, L. P. (2012). Questioning the rise of gelatinous zooplankton in the world’s oceans. BioScience, 62(2), 160-169. https://doi.org/10.1525/bio.2012.62.2.9
Diaz, R. J., & Rosenberg, R. (2008). Spreading dead zones and consequences for marine ecosystems. Science, 321(5891), 926-929. https://doi.org/10.1126/science.1156401
Falkowski, P. G., Barber, R. T., & Smetacek, V. (1998). Biogeochemical controls and feedbacks on ocean primary production. Science, 281(5374), 200-206. https://doi.org/10.1126/science.281.5374.200
Food & Drug Administration (FDA). (2023). National Shellfish Sanitation Program (NSSP) Guide for the Control of Molluscan Shellfish (2023 Revision). U. S. Food and Drug Administration. https://www.fda.gov/media/181370/download?attachment
Glibert, P., Anderson, D., Gentien, P., Granéli, E., & Sellner, K. (2005). The global, complex phenomena of harmful algal blooms. Oceanography, 18(2), 136-147. https://doi.org/10.5670/oceanog.2005.49
Gobler, C. J. (2020). Climate change and harmful algal blooms: Insights and perspective. Harmful Algae, 91, 101731. https://doi.org/10.1016/j.hal.2019.101731
IPCC. (2023). Climate change 2023: Synthesis report. Contribution of Working Groups I, II and III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Intergovernmental Panel on Climate Change (IPCC). https://doi.org/10.59327/IPCC/AR6-9789291691647
Jansen, A. (2023, August). Oysters as a Keystone Species in the Chesapeake Bay. Smithsonian Ocean. https://ocean.si.edu/ocean-life/invertebrates/oysters-keystone-species-chesapeake-bay
Keeling, R. F., Körtzinger, A., & Gruber, N. (2010). Ocean Deoxygenation in a Warming World. Annual Review of Marine Science, 2, 199-229. https://doi.org/10.1146/annurev.marine.010908.163855
Menzel, A., Sparks, T. H., Estrella, N., Koch, E., Aasa, A., Ahas, R., Alm-Kübler, K., Bissolli, P., Braslavská, O., Breide, A., Chmielewski, F. M., Crepinsek, Z., Curnel, Y., Dahl, Å., Defila, C., Donnelly, A., Filella, Y., Jatczak, K., Måge, F., … Zust, A. (2006). European phenological response to climate change matches the warming pattern. Global Change Biology, 12(10), 1969-1976. https://doi.org/10.1111/j.1365-2486.2006.01193.x
NASA. (2025, September 25). Ocean warming – Earth indicator. NASA Science. https://science.nasa.gov/earth/explore/earth-indicators/ocean-warming/
Oliver, J. D. (2005). Wound infections caused by Vibrio vulnificus and other marine bacteria. Epidemiology and Infection, 133(3), 383-391. https://doi.org/10.1017/s0950268805003894
Pinsky, M. L., Worm, B., Fogarty, M. J., Sarmiento, J. L., & Levin, S. A. (2013). Marine taxa track local climate velocities. Science, 341(6151), 1239-1242. https://doi.org/10.1126/science.1239352
Pörtner, H. O., & Knust, R. (2007). Climate change affects marine fishes through the oxygen limitation of thermal tolerance. Science, 315(5808), 95-97. https://doi.org/10.1126/science.1135471
Purcell, J. E. (2005). Climate effects on formation of jellyfish and ctenophore blooms: A review. Journal of the Marine Biological Association of the United Kingdom, 85(3), 461-476. https://doi.org/10.1017/s0025315405011409
Richardson, A. J., Bakun, A., Hays, G. C., & Gibbons, M. J. (2009). The jellyfish joyride: Causes, consequences and management responses to a more gelatinous future. Trends in Ecology & Evolution, 24(6), 312-322. https://doi.org/10.1016/j.tree.2009.01.010
Shumway, S. E. (1990). A review of the effects of algal blooms on shellfish and aquaculture. Journal of the World Aquaculture Society, 21(2), 65-104. https://doi.org/10.1111/j.1749-7345.1990.tb00529.x
Trainer, V. L., Bates, S. S., Lundholm, N., Thessen, A. E., Cochlan, W. P., Adams, N. G., & Trick, C. G. (2012). Pseudo-nitzschia physiological ecology, phylogeny, toxicity, monitoring and impacts on ecosystem health. Harmful Algae, 14, 271-300. https://doi.org/10.1016/j.hal.2011.10.025
Zu Ermgassen, P. S., Spalding, M. D., Grizzle, R. E., & Brumbaugh, R. D. (2012). Quantifying the loss of a marine ecosystem service: Filtration by the eastern oyster in US estuaries. Estuaries and Coasts, 36(1), 36-43. https://doi.org/10.1007/s12237-012-9559-y