METHODS
Site description
The ULE system offers a natural template for understanding the effect of
habitat heterogeneity on the spread of E. canadensis (i.e.
testing invasion hypothesis 3 ), being composed of a large
(surface area 3450 ha), generally shallow (mean water depth 2.3 m)
central lake (ULE), fed by the River Erne, which is linked by winter
floods and various channels and tributaries, to a network of over 50
small (<40 ha), shallow (<5 m) satellite lakes
(loughs; Fig. 1). The system has been subjected to eutrophication since
the 1950s (Battarbee 1986; Zhou 2000) with current annual concentrations
of total phosphorus (TP) and total nitrogen (TN) ranging between 29-383
μg/L and 0.22-2.25 mg/L respectively. E. canadensis is thought to
have colonized the ULE system in the 1880s (Simpson 1984). Records from
the Botanical Society of Britain and Ireland (BSBI) and more recent
macrophyte monitoring programmes indicate presence of E.
canadensis in ULE and its satellite lakes from the 1950s (Table S1).
The zebra mussel (Dreissena polymorpha ) also invaded this system
in the 1990s, resulting in strong phytoplankton biomass reductions and
increases in water transparency in the ULE (Minchin et al. 2003).
Macrophyte sampling and diversity predictors
Macrophytes were sampled in four basins within the central ULE and in 13
satellite lakes representing a gradient of nutrient-enrichment, zebra
mussel occurrence and hydrological connectivity to the ULE (Fig. 1). The
lakes were grouped into three categories according to Salgado et al.
(2019) as: Group 1 included the ULE and lakes directly connected
to it through the River Erne (Castle, and Derrykerrib) or via
tributaries (Doo and Mill). These lakes are diverse (macrophyte
richness= 17.4 ± 2.7), meso-eutrophic (TP= 55.1 ±11.3 μg/L), and have
clear waters (secchi depth= 222 ±47 cm) and high occurrence of zebra
mussels. Group 2 lakes are connected to ULE by flows through
intermediate lakes and associated tributaries (Killymackan, Cornabrass
and Kilturk). The lakes are diverse (macrophyte richness= 18.7 ±4.6),
eutrophic (TP = 136 ±54.4 μg/L; secchi depth= 182.3 ±55 cm) and zebra
mussels occur sparsely. Group 3 lakes (Head, Digh, Derryhowlaght
and Gole) are more isolated than Group 2 lakes due to intervening
small hills, woodlands and roads (Figure 1). These lakes are the least
diverse (macrophyte richness= 9 ±3.5) and most eutrophic (TP = 176.8 ±
89.3 μg/L) and turbid (secchi depth= 113.3 ± 79.3; zebra mussels rarely
occur).
Sampling was undertaken in 1m2 units, approximating to
the plant neighbourhood scale (i.e. where individual native plants may
compete with E. canadensis ). The lake volume infested by
macrophytes (PVI) method of Canfield & Jones (1984) was used to
characterize the distributions and abundances of native macrophytes
(including charophytes, bryophytes and vascular plants) and of E.
canadensis . Macrophytes were surveyed during the summers of 2008-09 at
individual points from a boat by zig-zagging across the entire lake
using grapnel sampling and visual observations with a bathyscope. At
each sampling point we recorded latitude/longitude, water depth, average
plant height, and species cover (%). PVI was calculated at each point
as: (macrophyte cover x average height of macrophyte)/water
depth. For comparisons with previous monitoring data (Table S1) and to
assess E. canadensis abundance patterns at the lake scale,
percentage of occurrence of E. canadensis at each lake was also
calculated by dividing the number of sampling points at which E.
canadensis was observed by the total number of sampling points within
the lake X 100.
The number of macrophyte sampling points varied according to lake size.
A minimum of 30 points was sampled for the smaller (< 10 ha)
lakes (Loughs Doo, Gole, Digh, Gole and Derryhowlaght) and between 60-80
points for the remaining larger (12-30 ha) satellite lakes (Fig 1). The
ULE was sampled across four separate lake zones (~30
ha): Crom State (54.161731, -7.436668), Lisnaskea Rd.–Trannish
(54.225036, -7.475074), Derrylea Rd.–Trannish (54.218312, -7.469581),
and Smith Strands–Trannish (54.204659, -7.480395). A total of 20
sampling points per ULE zone were surveyed and a total of 540 sampling
points across all the study lakes were covered. While our sampling
approach missed some macrophyte species known to occur in individual
lakes, it nevertheless provides a useful representation of variation in
macrophyte distributions and abundances for the majority of species at
the plant neighbourhood scale (Salgado et al. 2018b).
Abiotic data
Our previous studies of the ULE system demonstrate that macrophyte
communities are structured by lake water transparency, which is
negatively related to nutrient concentrations (TP and TN) and positively
correlated with zebra mussel occurrence (Salgado et al. 2018a, Salgado
et al. 2019). Accordingly, we used water clarity as a predictor of
environmental stress variation across the macrophyte sampling points. An
index of water clarity for each sampling point was defined as: lake
secchi depth measured at the deepest point of each lake/water depth at
the sampling point.
Plant macrofossil data
Previously published plant macrofossil abundance data derived from dated
sediment cores were used to represent macrophyte community changes over
the last c. 120 years (Salgado et al. 2018a,b; Salgado et al.
2019a). Cores were taken from Castle Lough and the Trannish area of ULE
(lake Group 1 ), from Cornabrass and Killymackan (Group 2 ),
and from Gole and Head (Group 3 ). The plant-macrofossil data are
available from the Dryad Digital Repository:https://doi.org/10.5061/dryad.3jj548d.
E. canadensis remains preserve poorly in lake sediments (Davies
1985) and so we inferred its temporal abundance from a recent macrophyte
study in the ULE system (Salgado et al. 2019) and from available
historical monitoring data (Table S1). Salgado et al. (2019)
showed that macrophyte assemblages now found in the ULE or
closely-connected lakes (e.g. Castle and Derrykerrib) are similar to
those characterised in sediment cores prior to eutrophication (i.e. pre
1950s). However, macrophytes currently found in the more isolated
eutrophic sites (e.g. Gole and Head) resembled those characteristics of
sediment cores post eutrophication (i.e. post-1960). Accordingly, we
used the current E. canadensis occurrence data in lakes close to
the ULE as a surrogate of E. canadensis abundances prior to 1950
and the E. canadensis occurrence data for the isolated lakes to
infer E. canadensis abundances during the onset of eutrophication
from 1960-1980. More recent (post-1980) E. canadensis abundance
data were obtained from macrophyte surveys conducted in 1988, 2006 and
2009 (Table S1). To standardise these various sources of E.
canadensis occurrence into a single comparable abundance measurement,
we assigned the data to the following abundance categories: 5
(100%–80% occupancy of sampled points); 4 (79%–60%); 3
(59%–40%); 2 (39%–20%); 1 (19%–1%); 0 (0%).
Statistical analysis