Estuaries are transitional ecosystems between land, freshwater, and the ocean. Geomorphologically and hydrologically they are very diversified and their definition - basically, what is and what is not an estuary - is still a matter of scientific debate. Most estuaries suffer the influence of the tide and all receive, in amounts that differ according to the climatic region and season, an input of fresh water derived from land drainage, meaning that there is usually a salinity increase from the upper reaches (usually, the river) to the inlet. Depending on the amount of fresh water discharged through the estuary a continuous of conditions can be found, from tropical estuaries where in several cases the amount of fresh water discharged is so great that a dilution of seawater only occurs outside the inlet, to some systems in warm temperate and subtropical dry areas where freshwater input can be seasonally so small that a negative salinity gradient occurs (i. e. the salinity increases upstream from the ocean).
 
The reasons for the high productivity of estuaries are basically three (not necessarily by the order presented below). One is their shallowness, which implies that the whole water column and the bottom can receive enough radiation to support photosynthesis by primary producers. This enables the occurrence of a diversified set of plant associations: phytoplankton, microphytobenthos (unicellular algae that live on the surface of intertidal flats), seagrass beds, algal beds, saltmarshes (in temperate estuaries) and mangrove forests (in tropical estuaries). Another is the input of inorganic nutrients from the rivers that feed, together with sunlight, photosynthesis by the primary producers, and of dissolved organic matter, which provides abundant material for bacterial growth. Finally, the action of the tides redistributes nutrients and materials along the estuary and provides an additional supply of nutrients and organic matter from the ocean.
 
The estuaries that we know today are very recent ecosystems, geologically speaking. They result from the invasion of the beds of rivers (which have been running for millions of years in their present location) by the rise of sea level that started some12.000 years ago following the end of the last glacial period. Humans have selected estuaries as preferred settlement places, because they provide abundant food, shelter and gateways for navigation along the coast. They have been profoundly changed by human action during the last 1000-2000 years, through watershed modification, agriculture, land reclamation and bank modification, nutrient enrichment from agriculture and domestic origins, pollution and exploitation of biological and mineral resources.
 
Still, in spite of twenty-two of the thirty-two largest cities in the world being located around estuaries, they keep providing important ecosystem services. Take care of them!
 
The photographs you see here are from the Tejo and the Ria the Aveiro, two of the largest estuarine systems of the Iberian Peninsula.