Explore the River:

Shaping Ecosystems

Biotic aspects

Biotic means "living." The biotic components of an ecosystem are the living organisms that inhabit it. These components could include producer organisms, consumer organisms, and decomposers. The key biotic components of the zebra mussel's environment include algae (chlorophyll), bacteria and small zooplankton (which the mussels may consume), and predators such as fish, crabs and birds.

Let's take a closer look at two of these biotic factors: zebra mussels and phytoplankton.

Zebra mussels

This graph shows the number of zebra mussels occupying the bottom of the Hudson River, in units of numbers of mussels per square meter (zm/m2). Zebra mussels thrive on a hard surface or substrate like underwater rocks, and do less well on soft surfaces or substrates like muddy bottoms.

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Each year, scuba divers in the Hudson River randomly pick rocks from the hard or rocky areas of the river bottom. They count the number of mussels attached to the rocks, and measure their shell sizes. Scientists use a device called a benthic grab to look for zebra mussels in "soft-bottom" areas, and they count the mussels they find in these samples too.

Since scientists know approximately how much of the river bottom is rocky and how much is soft, they can combine these averages for an annual estimate of the total number of mussels in the freshwater portion of the river, as well as the average per unit of river bottom.

This graph shows that substantial numbers of zebra mussels were first observed in the Hudson River in 1992. At that time the average population size was 4,000 per square meter of river bottom. We can see that since then the numbers have changed each year, from a high of 4,000 mussels/m2 to a low of about 500 mussels/m2 in 2003… (step the viewer through the sample graph)

Phytoplankton

In the Hudson River, the predominant producer organisms are phytoplankton and rooted aquatic macrophytes (large, plant-like structures that grow from the river bottom in shallow areas). Phytoplankton are microscopic organisms that carry out photosynthesis while floating or drifting in the water column. Many consumer organisms eat phytoplankton, making them an important part of the food chain.

Chlorophyll gives many types of producer organisms — which include plants, algae and some types of bacteria — their green color. Chlorophyll is vital for photosynthesis, the process by which producers convert sunlight to energy. It is an indicator of the abundance of these producer organisms.

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Chlorophyll concentration is used to measure phytoplankton abundance because it's produced by phytoplankton and is easy to measure. In order to determine the amount of phytoplankton in a water sample, scientists filter out the plankton particles and extract and measure the amount of chlorophyll they contain.

This example shows that chlorophyll in the Hudson River varies between 44.28 mg/L and 0.27mg/L. Concentrations are highest in the summer, when daylight is most abundant and temperatures are highest.

Abiotic aspects

Abiotic means "non-living." The abiotic aspects of an ecosystem are its non-living chemical and physical characteristics. These factors affect the kinds of organisms that can inhabit a given ecosystem, and their abundance. Abiotic factors in an aquatic environment like the Hudson River include the temperature of the water, how much oxygen it contains, how acid or basic it is (pH), how fast or slowly it moves, and how much sunlight penetrates the surface. Additional factors include how much suspended sediment the water contains and its nutrient concentrations (nitrogen and phosphorus).

Let's take a closer look at a few of these abiotic factors.

Temperature

this affects the metabolic rate of organisms. (Metabolism is the set of chemical reactions within organisms that keeps them alive, such as digestion. Their metabolic rate affects their health and growth.) Temperatures fluctuate in the short term as weather changes, over the longer term as seasons change, and over even longer periods as climate changes. The life cycle stages of many organisms change with the season, as do air and water temperatures and the number of hours of daylight.

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In this example, which measures temperature in Celsius… (step the viewer through the sample graph)

Dissolved oxygen

Organisms in aquatic environments must adapt to lower concentrations of oxygen than organisms directly exposed to air. This is because O2 must be dissolved in the water to reach them, and water holds nowhere near as much oxygen as does air in Earth's atmosphere. (We're talking here about the kind of oxygen that organisms, including humans, are able to breathe (diatomic oxygen gas, or O2). The oxygen atoms in the water molecule (H2O) are not available for respiration.)

Dissolved oxygen gas (DO) is measured in milligrams O2 per liter (mg/L), which is equivalent to parts per million (ppm), a unit of measurement commonly used when the relative value is very small. At 20°C, river water saturated with O2 holds 9.1 mg/L (or 9.1 ppm) — only 0.00091% of water by weight. By contrast, the atmosphere is 20.9% oxygen!

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When oxygen in water is in equilibrium with the atmosphere at a specific temperature, we say it is 100% saturated. The colder the water, the more oxygen can dissolve in it before it reaches saturation. If the water we described above were to be cooled to 10°C, it could hold up to 11.3 mg/L O2 — 25% more oxygen than at 20°C. If the dissolved oxygen concentration of water is below 2 mg/L (or ppm), conditions are "hypoxic" and can stress aquatic organisms.

A number of factors affect the amount of dissolved oxygen in aquatic environments. Producers release O2 during photosynthesis, which can cause oxygen concentrations to be higher in the day than at night. Consumers take up oxygen during respiration, which also influences concentrations. So can temperature fluctuations.

Suspended solids

Total suspended solids (TSS) refers to the solid particles that are suspended in water, which is an important indicator of water quality. TSS is measured by pouring a water sample through a pre-weighed filter. Material too large to pass through is considered "particulate" (or a suspended solid), while the material that passes through the filter is considered "dissolved." The amount of suspended solids is determined by drying and weighing the filter that contains the trapped particles.

TSS may be composed of both biotic particles (such as phytoplankton) and abiotic particles (such as silt and clay). TSS is important to aquatic producer organisms because suspended particles scatter and absorb sunlight, which affects the amount of light available for photosynthesis. TSS also affects many consumer organisms because some portion may be edible. Also, if there's too much particulate matter in the water, it can harm the many aquatic organisms that filter feed, such as mussels and other bivalves.