Wednesday, January 21, 2015

Intermittent vs Perennial streams in the Mediterranean

A very rare near-pristine perennial stream in mid June in Western Turkey (Alabayir Village, near Cine on the Cine river catchment); A single species of fish is present. Note that few deciduous trees and shrubs are in the riparian zone. 

Stream flow intermittency: an elusive concept

Most people don't consider the one-and-most important attribute defining stream ecosystems: the states of intermittent versus perennial flow regime. I think the Australians call this issue "ephemerality of flow". Its a big topic in Med and semi-arid climates. Low or no flow periods during summer are typical in the Mediterranean climate regions; they are a key ecological limiting factor in these ecosystems. There seems to be a threshold and break point. After regular dessication for a few months these river systems are different: intermittent. They are therefore considered temporary rivers - a wide variety of temporary rivers exist.

Prior modification by humans, many intermittent or ephemerally flowing streams where in fact perennial! Very few people know about this (or seem to care). So what we see a lot of today are "artificially intermittent streams" due to anthropogenic pressures (i.e. over-pumping of aquifers, tapped springs, over-abstraction of surface waters, dams etc).

Streams are varied, many types exist in the Med.
Many small catchments are really desiccated naturally by local climatic conditions: Lengthy drying periods that occur naturally and make rivers bone-dry for a long time (over 4 months, for example). These are naturally intermittent. When there is no groundwater feeding, no wet refugia for wholly aquatic life we have very few wholly aquatic biota that can adapt. These naturally intermittent streams are interesting, full of wonder and semi-aquatic life but they are different from perennial systems: fewer hygrophilous deciduous trees, fewer and hardier fishes, different plant communities, different species of water bugs; many terrestrial species living withing the river-bed.

An artificially intermittent stream in Western Turkey, near Karpuzlu; here most of the water is pumped away for agriculture and large sections are dry for a for a few months. Summer 2014.

A small intermittent stream in Eastern Central Greece, near Lamia in early June. One species of fish, Sperchios Barbel present in tiny pools. The fish migrate up and down when the stream floods in winter-spring. Summer 2014.

The above stream 4 km downstream of the above site; near Lamia, Greece. Bone-dry by early June; no fish. Is it naturally or artificially intermittent? The big trees are Oriental Planes good indicators that there is ground water. There may be a wet hyprhoeic zone beneath the stream bed. Summer 2014.
The Xeropotamos river on Samothraki Island - it is a naturally intermittent stream (now totally intermittent due to increased water diversions for agriculture). The river bed is wide and a fascinating rocky habitat for local terrestrial biota. This photo is from late July. Note the near-lack of deciduous trees on the riparian. Summer 2014.
The Erassinos stream of Attika Greece (near Vravrona outside of Athens) in February 2014. This stretch is an artificially intermittent part with no fishes and reduced riparian vegetation. Upstream - about 3 km - are the rivers springs which maintain water and fishes year-round.

Fishing in a small intermittent stream in Southeast Cyprus, the Oroklini stream just south of the National Highway between Larnaka and Pyla. The stream is now artificially perennial due to the increased waters from sprawl in its small catchment! Mosquitofish and eels exist at this section (just before Oroklini Lake).Winter 2012.

Definitions for education's sake:

Perennial Stream – A well-defined channel that contains water year-round during a year of normal rainfall. Aquatic environment of river bed located below the water table for most of the year. Groundwater is the primary source of water for a perennial stream, but it also carries storm-water runoff. The flow may be heavily supplemented by storm-water runoff. Hydrological and physical characteristics commonly associated with the continuous conveyance of water.

Intermittent Stream – A well-defined channel that contains water for only part of the year, typically during winter and spring when the aquatic bed is below the water table. Water is absent - in a "bone-dry" conditions for at least three months for a considerable stretch of the stream (in a normal year). Entire river sections (tens of kilometers normally are dry in summer). Usually the riparian vegetation and aquatic vegetation are different in a similar stream with Perennial character. An intermittent stream often lacks the biological and hydrological characteristics commonly associated with the perennial conveyance of water. 

Ephemeral Stream – Ephemeral stream means a feature that carries only storm-water in direct response to precipitation with water flowing only during and shortly after major precipitation events. An ephemeral stream may or may not have a well-defined channel, the aquatic bed is always above the water table, and storm-water runoff is the primary source of water. An ephemeral stream  lacks the biological, hydrological, and physical characteristics commonly associated with the perennial or intermittent conveyance of water. In the Mediterranean deciduous trees are usually absent on ephemeral stream riparian zones.

I'll keep you posted about this topic in the near future.

  • Check this link for a general development from the states:

Intermittent vs Perrenial Stream Bioassessment Arizona USA

  • For a recent European approach developed within the MIRAGE project which HCMR also took part in please see:

A novel approach to analysing the regimes of temporary streams in relation to their controls on the composition and structure of aquatic biota

  • Another integrated presentation of classification approaches again from the MIRAGE project which HCMR also took part is: