Aquatic ecosystems, encompassing diverse environments like oceans, lakes, rivers, and wetlands, are fundamental to global biodiversity and play a role in climate regulation. These complex systems are dynamic and constantly influenced by various environmental conditions. Understanding the factors that shape and sustain aquatic life is important for comprehending their health and functioning.
Light
Sunlight serves as the primary energy source for nearly all aquatic ecosystems. This energy fuels photosynthesis, a process carried out by primary producers like phytoplankton and aquatic plants, forming the base of the aquatic food web. These producers convert carbon dioxide and water into organic matter, supporting a vast array of aquatic life.
Light penetration varies significantly due to depth, turbidity (cloudiness from suspended particles), and dissolved substances. As light travels deeper, its intensity decreases, with longer wavelengths like red and orange absorbed more quickly than shorter blue and green wavelengths. This creates distinct zones: the photic zone, where light supports photosynthesis, and the aphotic zone, a lightless region below. The photic zone’s depth ranges from centimeters in turbid waters to over 200 meters in clear oceans. This light stratification directly influences organism distribution and ecosystem productivity.
Temperature
Water temperature profoundly influences aquatic ecosystems by directly affecting the metabolic rates of organisms like fish, invertebrates, and microbes. As temperature increases, the metabolic rate of cold-blooded aquatic animals rises, increasing their need for food and oxygen. Conversely, colder temperatures slow metabolic processes, reducing activity and food requirements.
Temperature also affects the solubility of gases, especially dissolved oxygen (DO). Colder water holds more DO than warmer water, which is important for aquatic life, as warmer waters can lead to reduced oxygen availability. Significant temperature variations, whether seasonal, daily, or from thermal pollution, can lead to thermal stratification in lakes and oceans. This layering, with warmer, less dense water at the surface and colder, denser water below, can impede vertical mixing, affecting nutrient cycling and species distribution. Temperature extremes cause physiological stress, impair function, and can lead to mortality, as many species have specific temperature ranges for survival and reproduction.
Nutrient Availability
Key nutrients, primarily nitrogen (N) and phosphorus (P), are important for aquatic life, serving as fundamental building blocks for primary producers like algae and aquatic plants. Carbon and various trace elements are also necessary for their growth and survival. The availability of these nutrients directly influences ecosystem productivity, as they are necessary for photosynthesis.
Nutrients enter aquatic systems from both natural and human sources. Natural sources include rock weathering and organic matter decomposition. Human activities significantly contribute through agricultural runoff (fertilizers), wastewater discharge, and sewage. Excessive nutrient enrichment, known as eutrophication, can lead to rapid, harmful algal blooms. When these dense algal populations die and decompose, they consume large amounts of dissolved oxygen, leading to oxygen depletion and “dead zones” that harm aquatic life.
Dissolved Oxygen
Dissolved oxygen (DO) is necessary for the respiration of most aquatic organisms, including fish, invertebrates, and decomposers. Animals extract this molecular oxygen, not the oxygen bound in water molecules, to fuel their metabolic processes. Primary sources of DO are atmospheric diffusion and photosynthesis by aquatic plants and algae during daylight hours. Rapidly moving water, such as in streams or areas with wave action, has higher DO levels due to increased atmospheric exchange.
Several factors influence dissolved oxygen levels. Water temperature is a major determinant, with colder water holding more oxygen than warmer water. Salinity also plays a role, as DO is less soluble in saltier water. Organic matter decomposition by microorganisms consumes oxygen, substantially reducing DO levels, especially after large algal blooms. Low DO levels, termed hypoxia (less than 2-3 mg/L) or anoxia (complete absence of oxygen), create stressful conditions for aquatic life, leading to impaired growth, increased disease susceptibility, and mass mortality.