The abyssopelagic and abyssobenthic zone of the ocean, the largest biome on this planet, is in perpetual darkness. Yet production in these zones is driven largely by seasonal plankton blooms at the surface. Describe this system in rough terms and why season affects it. What does the Sverdrup model have to do with
The abyssopelagic and abyssobenthic zones of the ocean are indeed the largest biome on Earth and exist in extreme depths where sunlight cannot penetrate, resulting in perpetual darkness. Despite the absence of light, the production of organic matter in these zones is still influenced by the seasonal plankton blooms occurring at the surface.
To understand this system, let's start with the concept of marine food webs. The base of these food webs is formed by phytoplankton, which are tiny plants that perform photosynthesis. Phytoplankton require sunlight for photosynthesis, so they are primarily found in the sunlit upper layers of the ocean where they can access enough light.
During certain times of the year, environmental conditions such as temperature, sunlight availability, nutrient availability, and water column stability become favorable for phytoplankton growth at the ocean's surface. This leads to an increase in phytoplankton populations, known as a plankton bloom, where they multiply rapidly and spread across the sunlit layer.
As plankton blooms occur at the surface, they generate an abundance of organic matter through photosynthesis. This organic matter can then sink down through the water column due to various factors like gravity, water circulation, and sinking behavior of individual organisms. As it sinks, the organic matter serves as a food source for organisms residing in the abyssopelagic and abyssobenthic zones.
The sinking organic matter can be consumed by organisms in these deep zones or it can be broken down by bacteria through a process called remineralization. This transfer of energy and nutrients from surface waters to the deep ocean is known as the biological pump.
Now, let's talk about the Sverdrup model. The Sverdrup model, named after the oceanographer Harald Sverdrup, is a conceptual model used to explain the dynamics of plankton blooms in the ocean. According to this model, the growth of phytoplankton depends on the availability of essential nutrients such as nitrogen, phosphorus, and iron. These nutrients are often limited in the surface ocean, so the Sverdrup model emphasizes the role of nutrient upwelling from the deeper ocean layers to stimulate plankton growth.
During the summer months, increased sunlight and warmer surface waters lead to stratification, creating a barrier between nutrient-rich deep waters and the sunlit surface waters. This stratification inhibits nutrient mixing and availability in the upper ocean, limiting the growth of phytoplankton. As a result, phytoplankton populations remain relatively low during this time.
However, as the seasons change, the surface waters become cooler and wind patterns can change, leading to an increase in mixing between surface and deep waters. This mixing brings up nutrient-rich waters from the depths to the sunlit surface, providing the necessary nutrients for phytoplankton to thrive. Consequently, favorable conditions for plankton blooms are created, leading to an increase in organic matter production.
In summary, the abyssopelagic and abyssobenthic zones of the ocean depend on the seasonal plankton blooms occurring at the surface for their energy and nutrient sources. The Sverdrup model helps explain how nutrient availability and mixing in the water column play a crucial role in driving these seasonal variations in phytoplankton growth and subsequent organic matter production.