Primary Production

Primary Production and Primary Producers

In this lecture, we will explore the concepts of primary production and primary producers. This discussion will overlap with previously discussed topics such as nutrient limitation and stream metabolism, and will further be elaborated in subsequent lectures. The approach will help reinforce key ideas and perspectives.

Lecture Structure

The lecture is divided into two main parts. By the end of this session, and following the assigned readings, students should be able to:

  1. Refresh their understanding of the basics of photosynthesis—an essential process for understanding primary production and its role in stream ecosystems.
  2. Distinguish between Net Primary Production (NPP) and Gross Primary Production (GPP). Although metabolism work often emphasizes GPP, discussions around production and food web dynamics will primarily focus on NPP.
  3. Comprehend how changes in dissolved oxygen (DO) can be utilized to estimate GPP, starting with closed chamber contexts before transitioning to open channel metabolism estimates.
  4. Understand the interplay of light and nutrients on primary production, along with the implications for standing stocks in streams.

Algae in Streams

Forms of Benthic Primary Producers

There is a diverse array of benthic primary producers in streams. In stream ecology, significant emphasis is placed on studying both macroinvertebrates and primary production within the benthic zone, referred to as the benthos. This term is frequently used in scientific discourse, notably by the Society for Freshwater Science, which was previously known as the North American Benthological Society. The society focuses on the benthic environments in freshwater ecosystems, which hold paramount ecological importance.

Types of Algae

Different forms of algae can be categorized based on their shapes and sizes, influencing their vulnerability to various grazers (macroinvertebrate functional feeding groups). These groups consist of:

  • Gatherers or Shredders: These organisms primarily consume larger filamentous materials.
  • Scrapers: These organisms feed on smaller, less complex algal forms.
  • Rasping Scrapers: This group targets algae such as diatoms found on the surfaces of rocks.

In streams, primary production may seem minimal upon casual observation (e.g., rocks in a stream), yet significant activity often occurs, albeit at low rates or under heavy grazing pressure. Common forms of algae present might include bryophytes, stalked or filamentous algae, and gelatinous forms such as Nostoc, which is a colonial nitrogen-fixing cyanobacterium. Although macrophytes (larger aquatic plants) are not the primary focus of today's discussion, it is acknowledged that they are crucial in the ecology of larger river systems and should not be overlooked.

Primary Production Focus

The lecture will specifically focus on periphyton, a term used interchangeably with biofilms. The concept of periphyton encompasses:

  • An assemblage of freshwater benthic photoautotrophic algae and prokaryotes, including heterotrophic bacteria and fungi that either feed on algae or on decaying organic matter (detritus) in freshwater streams.
  • It is critical to recognize that periphyton is a composite of organisms and not synonymous solely with algae. Therefore, calculations concerning the mass of periphyton often differ from merely measuring algae alone.

Importance of Primary Production

Primary production is fundamentally defined as the process by which energy is stored in chemical bonds through the reduction of carbon dioxide into carbohydrates, facilitated by the presence of light. This process is vital as it serves as the primary currency of ecosystems, with fixed carbon functioning as a source of potential energy.

The general equation for photosynthesis can be summarized as follows:
6CO2 + 6H2O + ext{light energy}
ightarrow C6H{12}O6 + 6O2
In this equation:

  • Carbon dioxide from the atmosphere combines with water to yield glucose (a simple sugar, C6H12O6) and oxygen (O2) as byproducts.

Charge Balance of Photosynthesis

In terms of charge balancing for photosynthesis, oxygen carries a charge of -2; with six oxygen atoms contributing a collective charge of -12. Twelve hydrogen atoms balance, and thus the carbon's charge concludes at +4 for CO2. After processing, the glucose results in a charge of 0 for carbon, signifying that carbon is reduced through this process, illustrating the potential energy change associated with fixed carbon.

Net Primary Production vs. Gross Primary Production

Definitions:

  1. Gross Primary Production (GPP): The total amount of carbon that is fixed by photosynthesis within a given timeframe.
  2. Net Primary Production (NPP): The equivalent amount of carbon fixed minus the carbon consumed through plant respiration.
    • Approximately 50% of the carbon fixed in GPP is utilized for plant respiration.

Empirical Data on NPP and GPP

Empirical studies demonstrate that NPP is generally about 50% of GPP, illustrated by a slope nearing 0.5 in relevant graphs showing carbon fixation rates.

Ecosystem Dynamics Sensitivity to NPP and GPP

NPP is crucial for sustaining other organisms within the system, as it represents the organic matter that fuels higher trophic levels, including both detritivores and herbivores, thereby contributing to net ecosystem production within the stream community. This forms the basis for calculations of Net Ecosystem Production (NEP), defined as GPP of all autotrophs minus total oxygen consumption across the ecosystem.

Metrics for Stream Health

Assessing ecosystem health is often approached via the ratio of production to respiration (P:R ratio) or the GPP to Ecosystem Respiration (ER) ratio, yielding insights regarding whether a stream is net autotrophic or heterotrophic.

  • Net Autotrophic Systems: Produce more organic matter than is consumed by heterotrophs.
  • Net Heterotrophic Systems: Consume more organic matter than is produced.

Measuring Stream Primary Production

Standing Stocks and Proxies

  • Two primary analytical methods for measuring stream primary production are examining chlorophyll-a concentrations (a key photosynthetic pigment) and assessing biomass (often reported as ash-free dry mass).
  • The determination of chlorophyll-a typically involves scraping rocks in the stream to collect algae, followed by extraction and measurement of chlorophyll-a concentration.
  • The calculation for biomass involves:
    1. Weighing the slurry after scraping, including both organic and inorganic material.
    2. Burning the organic matter to isolate inorganic sediments, providing a more accurate indicator of organic matter content.
  • While these measurements provide useful insights into stream ecology, it is essential to recognize that high primary production does not always correlate with high standing stocks due to variations in consumption rates.

Ratio Influences

The ratio of actual dry mass to chlorophyll-a concentration can vary due to different environmental factors such as light intensity and productivity. In low light conditions, the ratio is generally low, indicating a requirement for both higher biomass per unit chlorophyll and underrated growth dynamics.

Estimating GPP in Streams

Oxygen as a Metric

Due to challenges in measuring CO2 directly, the assessment of oxygen change becomes an integral method for estimating primary production in streams. However, changes in oxygen must account for both GPP and ecosystem respiration, as both processes influence dissolved oxygen levels along with atmospheric exchanges.

Chamber Experiments

Chamber methods are frequently employed wherein a sealed environment is created to measure the change in oxygen without exchange with the atmosphere. In these experiments:

  • The alterations in dissolved oxygen during the day reflect the balance between photosynthesis and respiration.
  • Nighttime decreases in oxygen provide insight into plant respiration when photosynthesis does not occur.
  • Ultimately, the analysis considers both daytime GPP (oxygen production) and nighttime ER (oxygen consumption) to yield estimates for GPP in streams.

In conclusion, this lecture outlined the foundational intricacies of primary production, the roles entities play in stream ecosystems, and how effective monitoring and measurement techniques are crucial for understanding the dynamic relationship between primary producers and their environmental interactions.