Problem of the Week

Chapter 26: Chemistry in the Environment

Problem of the Week

Phytoplankton
Phytoplanktons, microscopic unicellular or single-celled plants known as algae, are vital to the survival of all marine organisms. Diatoms, dinoflagellates, and coccolithophores are organisms that belong to this group that grow abundantly in oceans around the world and are primary producers. The development and survival of the marine food chain depends on phytoplankton. Phytoplankton depend on certain conditions to grow and they use carbon dioxide for photosynthesis. As these organisms carry out photosynthesis, they generate and sustain most of the oxygen we breathe, they also help regulate atmospheric carbon dioxide. The amount of carbon dioxide pulled from the atmosphere into the ocean depends on the size of the phytoplankton population.

Phytoplankton are good indicators of environmental change and are of primary interest to oceanographers and scientists around the world. These plants contain chlorophyll for photosynthesis. Chlorophyll is a pigment that absorbs red and blue light while reflecting green light. Depending of the type and density of phytoplankton populations, satellite photographs show regions of blue-green to green. Images with lots of green indicate large amounts of chlorophyll, which in turn implies healthy photosynthesis and growth. Growth of phytoplankton indicates the successful use of carbon dioxide.

Phytoplankton requires nutrients to live. The deep ocean waters are one source of nutrients but the mouths of rivers often supply tons of nutrient rich water into the ocean providing abundant resources for phytoplankton. Rivers and streams carry decomposing plant and animal matter as well as excess fertilizer into the ocean. In nutrient rich waters, phytoplankton grows in wide colonies called blooms. These blooms are highly variable in their nature; their size and intensity are significantly dependent on surrounding environmental conditions. Water must contain high levels of the nutrients nitrogen and phosphorus, have water temperature in a certain range, and have salinity in the optimal level to support an algal bloom. During the bloom, phytoplankton consume nutrients causing a decrease in the amount of dissolved nitrogen and phosphorus in the water body. Overall dissolved oxygen (DO) levels will generally increase, as phytoplankton produce oxygen as part of the photosynthetic process during the day; at night, DO levels decrease sharply as the algae consume oxygen. As the nutrients become depleted, the algae can no longer survive and populations decrease sharply in what is called a "crash." As this crash is occurring, the dead phytoplankton sink to the bottom of the water column, where they are consumed by decomposers. Since these decomposers require oxygen to break down the algae, dissolved oxygen levels will decrease during this time period. Resulting low oxygen levels can be detrimental to fish health; if DO drops to below 3 mg/l, fish kills can result.

Determine the molarity of oxygen in the ocean when the dissolved oxygen level is 3 mg/l.

Sources of nitrogen used in fertilizers are ammonia, NH₃, ammonium biphosphate, (NH₄)₂PO₄, ammonium nitrate, NH₄NO₃, ammonium sulfate, (NH₄)₂SO₄, and urea, N₂H₄CO. Determine the mass percent of nitrogen in each of the sources of nitrogen.

Coccolithophores are one-celled marine plants that live in large numbers throughout the upper layers of the ocean. Unlike any other plant in the ocean, these tiny organisms surround themselves with calcium carbonate. Scientists have estimated that coccolithophores dump 1.4 x 10⁹ kilograms of calcium carbonate into the ocean each year.

a. Determine the number of moles of calcium carbonate that these organisms dump into the ocean in one year.
b. Coccolithophores produce calcium carbonate from the following reaction:

CO₂ + H₂O → HCO₃- + H⁺ → CO₃^-2 + 2 H⁺
Ca²⁺ + CO₃^-2 → CaCO₃

Determine the mass of carbon dioxide absorbed from the atmosphere when the coccolithophores dump the 1.4 x 109 kilograms of calcium carbonate into the ocean each year.