Estuaries are eternally enriched. Their positions at the foot of
watersheds and their convenience as receiving bodies for the wastes of
cites, towns and farms results in continuous addition of nutrients -
those elements and compounds which are essential for organic production.
Such materials must be added to these complex bodies of water to sustain
production, since there is a net loss of water and its contents to the
oceans. Enrichment from land and the ocean and the subsequent cycling of
the original chemicals or their derivatives contribute to the
extraordinarily high values of estuaries for human purposes. Many
estuaries are able to assimilate large quantities of nutrients despite
the great fluctuations which occur with variations in the flow from
tributaries. The nutrients can be stored, incorporated in standing crops
of plants, released, cycled and exported - and the system frequently
achieves high production of plants and and animals without creation of
any undesirable results of enrichment. Excessive enrichment with the
same elements and compounds can, however, be highly detrimental to
estuaries and their uses. Coastal cities are usually located on the
estuaries which provided a harbor for the- and which now receive
partially treated sewage and other wastes from the expanding population
and industrial activity. Conversion of woodlands to agricultural use and
the extensive application of fertilizers have resulted in the flow of
large quantities of nutrients down the hill or slopes and eventually
into the estuary.
