Algal mats and insect emergence in rivers under Mediterranean climates: towards photogrammetric surveillance

Abstract

1. In a nitrogen (N)-limited river subject to Mediterranean summer drought hydrology, the colour of macroalgal proliferations changed with successional and seasonal changes in epiphyte assemblages. New growth of the dominant macroalga, Cladophora glomerata, was green, as were proliferations of Oedogonium, Mougeotia and Spirogyra, which did not become heavily colonized with diatoms. Green Cladophora turned yellow as Cladophora filaments became colonized by diatoms that were not N fixers, and turned rust-coloured as later-successional epiphyte assemblages became dominated by dense Epithemia turgida and E. sorex, which both contain N-fixing cyanobacterial endosymbionts. 2. The rate and composition of insect emergence from floating algal mats differed among proliferations of different colour. The rates of emergence (individuals day )1 500 cm )2 ) of nematoceran flies were three to 25 times greater from yellow or rusty-coloured Cladophora mats than from green Cladophora, Oedogonium or Mougeotia mats that had lower epiphyte densities. Biomass emergence from Cladophora mats that were rusty in colour was eight to 10 times greater than from yellow Cladophora mats, because larger nematocerans dominated in rusty mats (Chironominae versus Ceratopogonidae in yellow mats). 3. Proliferations of Epithemia-infested Cladophora occur at and above drainage areas of about 100 km 2 (channel widths of 25-30 m) in this river network, coinciding with the drainage area threshold where a step increase in concentration of total dissolved N is observed during summer. 4. In rivers under Mediterranean climate regimes, algal succession during the prolonged low flow season is less subject to stochastic interruption by spates than in rivers under more continental climates. Under these summer drought conditions, photogrammetric detection of colour changes in algal proliferations may help us track reach or basin-scale change in their ecological functions.

Midges,Cladophora, and epiphytes: shifting interactions through succession

“…Initially, retreat weaving by midges reduces Cladophora biomass through clipping and fragmentation (Power 1990a), but midge grazing grooms epiphytes from underlying Cladophora ( fig. 6 in Power et al 2009), enhancing its exposure to light and nutrient fluxes. These observations suggest a complex interaction between midges, Cladophora, and its associated epiphytes.…”

“…Epiphyte assemblages on Cladophora vary seasonally and spatially down the drainage network, e.g., as light, flow, and substrate change (Power 1990a, b, Bergey et al 1995, Power et al 2009). In early summer, new Cladophora growth is green with a light load of algal epiphytes, which generally include stalked diatoms, such as Gomphonema Ehrenb.…”

“…This unique group of diatoms contains N-fixing endosymbiotic cyanobacteria (Floener andBothe 1980, DeYoe et al 1992). Epithemia-covered Cladophora can elevate rates of N-fixation, increasing the biologically available N in the river (J. Welter, St. Catherine University, unpublished data), especially in reaches draining .100 km 2 , where Cladophora and its epiphytes dominate the summer biomass of primary producers (Power et al 2009). Spatial and temporal variation in densities and distribution of loosely vs tightly attached algal epiphytes and N-fixing taxa probably influence the quality and quantity of algal food available to grazers (McCormick andStevenson 1989, Dudley 1992).…”

“…Larvae of the herbivorous midge, Pseudochironomus richardsoni, weave retreats from Cladophora and graze on its algal epiphytes, primarily diatoms (Power 1991, Gresens 1997, Power et al 2009). Initially, retreat weaving by midges reduces Cladophora biomass through clipping and fragmentation (Power 1990a), but midge grazing grooms epiphytes from underlying Cladophora ( fig.…”

“…After winters with scouring floods, densities of a large voracious caddisfly, Dicosmoecus gilvipes (Hagen), are reduced, releasing the filamentous green alga, Cladophora glomerata (L.) Kü tz., from grazing pressure (Power 1990b(Power , 1992a. Subsequent proliferations of Cladophora filaments, often several meters in length, can increase the surface area available to algal epiphytes up to 200,0003 over plan-view (water surface) area (Power et al 2009). …”