Site, experimental design, methods, and data
The experiment was carried out at Ribeira do Botão, a
3rd order stream located near Coimbra, Central
Portugal (40°18′23″N, 08°23′55″W; elevation 80 m) during 56 days from
November to December 2011. The riparian vegetation was composed of mixed
deciduous trees, mainly common alder. During the exposure period (data
collected at a nearby meteorological station located at 40°12′33″N;
08°27′08″W; elevation 16 m), daily air temperature ranged 4.8–19.9°C,
humidity ranged 54%–96%, and precipitation totalled 241 mm. In the
stream, water was well-oxygenated (11.9-13.0 mg L-1),
temperature ranged 5.9–13.4⁰C, pH was circumneutral (6.6-7.3), nitrate
ranged 0.45-2.55 mg NO3--N
L-1, ammonia ranged 21.5-94.8 µg
NH4+-N L-1,
phosphate ranged 79.7-109.4 µg PO43--P
L-1, and current velocity ranged 0.239-0.458 m
s-1.
Other characteristics of the study site and the litter decomposition
study are described in detail in Abelho and Descals (2019). Briefly,
decomposition of senescent leaf litter (2.80 g ± 0.52 SD ) of the
N-fixing alder (Alnus glutinosa (L.) Gaertn.), hybrid black
poplar (Populus × canadensis Moench) and a 1:1 mixture of both
species was assessed on litter bags (5 mm mesh) exposed to both the
terrestrial and the aquatic environment according to three exposure
scenarios (Figure 1): 0:100 (0 days terrestrial followed by 56 days
aquatic), 25:75 (14 days terrestrial followed by 42 days aquatic), and
50:50 (28 days terrestrial followed by 28 days aquatic). The riparian
leaf bags were deployed on the floor next to the stream channel
(approximately 1.5 m above the stream surface and 1 m away from the
stream margin), and loosely covered with dead fallen leaves. The stream
bags were tied to iron nails fixed at the bottom in a riffle area. Three
additional similar portions of each litter type were used to determine
the initial oven‐dry mass (60°C, 3 days) of the leaves used in the
experiment. Sampling in stream (0:100) was carried out after 7, 14, 28
and 56 days of immersion. Sampling in the terrestrial environment
occurred after 14 (25:75) and 28 days (50:50) of exposure; three
replicates of each litter type were used to determine litter dry mass
remaining and litter colonisation in the terrestrial environment, and
the other groups were transferred to the riffle area of the stream and
sampled after 7, 14, 28 and 42 days (25:75) or after 7, 14 and 28 days
of immersion (50:50).
On each sampling occasion, mass loss of leaf litter (% dry mass),
ergosterol as an estimate of fungal biomass (mg
gAFDM–1), aquatic hyphomycete
sporulation rates (number of conidia
mgAFDM−1 day−1) and
taxa richness (number of species sample–1), shredder
biomass (mg bag–1), abundance (number of individuals
bag–1), and taxa richness (number of taxa
bag–1) were determined on three replicates as
described in Abelho and Descals (2019). In the mixture, mass loss and
the fungal variables were determined for each species; to obtain the
value in the mixture, mass loss was summed, and the fungal variables
were averaged.
The two leaf species differed significantly in their initial chemical
characteristics, decomposition rates and colonisation patterns by
decomposers and detritivores (Table 1), as determined by Abelho &
Descals (2019).