The earliest approaches to quantifying aquatic productivity were based on indirect measurements. Rather than measuring the metabolic process of photosynthesis directly, these foundational methods infer productivity rates by measuring proxies, such as the amount of existing biomass or the rate of nutrient consumption from the environment. While limited in their precision, these techniques were historically important in the development of the field and established the conceptual groundwork for subsequent, more direct approaches.

2.1 Standing Crop Analysis

The core principle of the standing crop method is the measurement of the total biomass of primary producers present per unit of area or volume at a specific point in time. As the longest-used method for estimating productivity, it provides a snapshot of biomass that has not been lost to respiration.

Specific techniques for measuring standing crop include:

• Direct Collection: Plankton can be gathered with nets and subsequently counted under a microscope or weighed to determine biomass. This approach is also used for periphyton (attached algae) and rooted aquatic plants.
• Pigment Analysis: The concentration of plant pigments, particularly chlorophyll a, serves as a proxy for producer biomass. Modern advancements allow for this analysis to be conducted via remote sensing from aircraft or satellites, enabling large-scale assessments.

The primary limitation of this method is that standing crop is not directly proportional to productivity. Changes in standing crop reflect the net effect of many biological and physical events. Consequently, the standing crop of a phytoplankton community may be greatly diminished by predation and water movement, while the photosynthetic rates of the surviving organisms remain high.

2.2 Nutrient Uptake Analysis

This method estimates production by measuring the decrease of essential inorganic nutrients from the water column over time. The logic is that the rate of nutrient depletion reflects the rate at which those nutrients are being incorporated into new organic matter by primary producers.

Historically, this approach was applied in early attempts to quantify marine productivity. The work of Atkins (1922, 1923) in the North Sea, for instance, used the decrease in carbon dioxide and phosphate to measure production. Later, Steel (1956) estimated the annual cycle of plant production in the same region by considering changes in the inorganic phosphate in relation to vertical mixing of the water mass.

However, this indirect approach has key weaknesses that limit its precision. Phosphorus, for example, is a difficult element to study because organisms often engage in luxury storage, accumulating it in excess of their immediate requirements for optimal growth. Furthermore, the concentration of nutrients is influenced by various other biological activities, making it difficult to isolate the signal of photosynthetic uptake. The imprecision caused by these confounding biological and physical factors created an urgent need for methods that could isolate and directly quantify the metabolic signal of photosynthesis itself.