Principles of Biological Oceanography: Reproduction, Trophic Systems, and Productivity

Exam Tip: When asked to identify a strategy, look at the environment. If the environment is unstable, think r (rapid). If the environment is stable and crowded, think K (carrying capacity/competition).

Trophic Systems and Control Mechanisms

Energy in the ocean moves through various trophic levels, starting from primary producers (like phytoplankton) and moving up to top predators. However, the direction in which this energy or "control" flows can be categorized in two ways: bottom-up and top-down control.

Bottom-Up Control

In a bottom-up system, the ecosystem is driven by the availability of resources at the base of the food web. For example, an increase in nutrient availability leads to increased phytoplankton productivity, which subsequently increases the biomass and productivity of every higher trophic level (zooplankton, fish, etc.).

Top-Down Control

In a top-down system, the ecosystem is regulated by consumers (predators). When predators increase in number, they depress the population of the trophic level they feed upon. This reduction in consumers can lead to an indirect increase in the next lower trophic level (the "trophic cascade").

Production, Biomass, and Turnover

One of the most striking differences between marine and terrestrial ecosystems lies in how they manage their "standing crop" of biomass versus their rate of production.

The P/B Ratio

The Production-to-Biomass (P/B) ratio is the ratio of annual production to the average annual biomass. It essentially measures how much "new" organic matter is being created relative to the amount of living matter present at any one time.

$\text{P/B Ratio} = \frac{\text{Annual Production}}{\text{Average Annual Biomass}}$

In pelagic marine ecosystems, the P/B ratio is typically much higher than in terrestrial forest ecosystems. This is due to several factors:

1. Rapid Turnover: Phytoplankton have extremely rapid turnover rates; they grow and are consumed almost as quickly as they appear.
2. Lack of Structural Biomass: Unlike terrestrial plants, which must invest massive amounts of energy into long-lived structural components like wood and bark, marine primary producers are almost entirely "active" metabolic tissue.

Microbial Productivity

In the ocean, we must distinguish between different types of primary production. While phytoplankton productivity (the conversion of light and nutrients into organic matter by algae) is a major driver, bacterial productivity also plays a critical role in the cycling of carbon and nutrients. Scientists often use biomass-normalized carbon uptake (or carbon assimilation rates) to compare the efficiency of these different groups of organisms.

Study Note: If you see a question asking why a forest has a lower P/B ratio than an ocean, remember: Wood. Trees store massive amounts of biomass that doesn't "turn over" quickly, whereas phytoplankton are eaten or die almost immediately.