Abstract
Aquatic birds may impact shallow ecosystems via organic and nutrient enrichment with feces. Such input may alleviate nutrient limitation, unbalance their ecological stoichiometry, and stimulate primary production. Herbivorous and piscivorous birds may produce different effects on aquatic ecosystems due to different physiology, diet and feces elemental composition. We analyze the effects of droppings from swans (herbivorous) and cormorants (piscivorous) on phytoplankton growth via a laboratory experiment. These birds are well represented in the Curonian Lagoon, where they form large colonies. As this lagoon displays summer algal hyper-blooms, we hypothesize an active, direct role of birds via defecation on algal growth. Short-term incubations of phytoplankton under low and high feces addition produces different stimulation of algal growth, significantly higher with high inputs of cormorant feces. The latter produces a major effect on reactive phosphorus concentration that augments significantly, as compared to treatments with swan feces, and determines an unbalanced, N-limited stoichiometry along with the duration of the experiment. During the incubation period, the dominant algal groups switch from blue-green to green algae, but such switch is independent of the level of feces input and from their origin. Heterotrophic bacteria also are stimulated by feces addition, but their increase is transient.
Highlights
Estuarine productivity can be stimulated by riverine inputs, run-off from agricultural land, point pollution sources and internal recycling
We investigate with a laboratory experiment the effects produced by feces from herbivorous and piscivorous birds on microbial activity and on phytoplankton growth and community composition in the water of the Curonian Lagoon, Lithuania
Results from our experiment suggest that feces from herbivorous and piscivorous aquatic birds may produce different effects on dissolved inorganic nutrient concentrations, their ecological stoichiometry, and phytoplankton growth rates, but not on algal community composition and bacterial enrichment
Summary
Estuarine productivity can be stimulated by riverine inputs, run-off from agricultural land, point pollution sources and internal recycling. These nutrient sources can be temporally separated and may be quantitatively different [1]. Diffuse inputs, including those from rivers and agricultural land, are strongly dependent on precipitation and may, undergo pronounced seasonal variations [2]. Concerning eutrophic shallow ecosystems, internal recycling is often the dominant mechanism sustaining primary production in the warm season, when biological components are more active [3,4,5]. The macrobiota may contribute to internal nutrient recycling and primary production but its role is comparatively understudied.
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