Biotic and Abiotic Factors Changing Kelp Population Dynamics
Kelp is a type of brown algae with over 1500 different species that can be found in most of our oceans, especially along coasts with high upwelling events. In large amounts over small regions, kelp serves as an ecosystem for living creatures ranging from the bottom of the ocean to the sea-surface. Because kelp can be up to 50 meters tall and has many fronds, it provides shelter to fish and mammals from strong currents and larger predators. Although it is not as effective as phytoplankton, kelp is also a primary producer and can reduce carbon dioxide levels in its local environment while being a source of food for species such as purple sea urchins, halfmoon fish, and opaleye. There is a delicate balance that is required to maintain this ecosystem, and therefore changes in kelp health, density, and population are of extreme importance.
Abiotic Factors that Affect Kelp Health, Density, and Population
Nutrient Availability
Not unlike terrestrial plants, kelp requires nutrients such as Nitrogen and Phosphorus to grow and reproduce. Seasonal differences in strength of coastal upwelling results in natural variations in nutrient concentrations. Furthermore, episodic El Nino events increase average temperatures and stratify the ocean, reducing the amount of water mixing and nutrient dispersal that occurs vertically in the ocean.
Temperature
Kelp favors cold, temperate regions, so regions where huge temperature variations develop or yearly average temperature is on the rise can become unsuitable environments for the species. There exists a natural varation in kelp growth due to seasonality, but when temperatures stay warm year round due to long summers or events like El Nino, kelp reproduction remains very low and often times can result in death of the species over the regions affected by prolonged warm climates.
Light
Light is essential for growth of kelp because it provides energy for photosynthesis. In addition to this, light availability can determine the production of gametes, thus having ties to the effectiveness of the reproduction cycle.
Strength of Currents
In small amounts, currents have the potential to transport nutrients into nutrient depleted kelp regions and favor their health. However, strong currents can also transport warmer waters from other regions into the kelp ecosystems and damage holdfast of the algae. When this happens, the algae is displaced from its habitat into unfavorable regions where it dies.
Biotic Factors that Affect Kelp Health, Density, and Population
Grazing
In recent years, there has been a surge in purple sea urchin populations resulting in urchin barrens which have limited biodiversity. Sea urchins eat kelp that has been damaged and sinks to the sea floor, but they also eat the steadfasts. When kelp density drops, the entire ecosystem changes because it serves as its foundation.
The rise in sea urchin populations along the U.S. West Coast can largely be accounted by the mass hunting of otters that occured in the 19th century. Sea otters are sea urchins’ largest predator and helped keep their numbers under control. There is an increasing interest in re-introducing sea-otters in Southern California’s Ocean because they consume purple sea-urchins and could reduce the emergence of sea-urchin colonies, but this has been met with opposition from fishermen because they worry that otters will also consume clams, mussels, and crabs.
As global temperatures, human population, pollution events, and overfishing increase, it will become increasingly important to thoroughly understand these effects on kelp. Although we are familiar with the biotic and abiotic factors affecting kelp resilience, it is also important to understand how they act in conjunction and over longer periods of time. In addition to this, most research focuses on adult giant and bull kelp, so the effects of grazing, nutrient depletion, etc on other species and different life stages are unknown. My research project aims to bring more knowledge to this topic through the use of an experiment where temperature, light, and nutrient availability vary by three different levels. In addition, this project will be repeated over the four major kelp groups and for kelp of different ages.
To gain more insight on biotic factors such as grazing, I propose selecting a small kelp region in the ocean of about 5x5 meters with a high, average purple sea urchin population and taking in-situ measurements of kelp density and health in the existing conditions. Once that has been done, most of the purple sea urchin population can be removed and changes in the measurements can be tracked daily for a period of six months. The overall hope for this research is that it will provide a deeper understanding of the role of kelp in ocean ecosystems and how that role will change as those ecosystems are affected by naturally-driven and human-led climate change.