Isotopic niche size of Coregonus artedi (sensu lato) Increases in the presence of Mysis diluviana, expanded habitat use and phenotypic diversity

Abstract

Post-glacial colonization of lakes in Algonquin Park, Ontario, Canada resulted in food webs with cisco (Coregonus artedi sensu lato) and either Mysis diluviana or Chaoborus spp. as the dominant diel migrator. Mysis as prey, its diel movements and benthic occupancy, are hypothesized to be key elements of ecological opportunity for cisco diversity in the Laurentian Great Lakes. If correct, the hypothesis strongly implies that lakes with Mysis would have greater trophic niche size and drive greater adaptive radiation of cisco forms relative to lakes without Mysis. The dichotomy in diel migrator in Algonquin Park lakes was an opportunity to assess the isotopic niche size of cisco (δ¹⁵N and δ¹³C) and determine if niche size expands with Mysis presence. We found the presence of Mysis is necessary to expand isotopic niche size in our study lakes. The use of habitats not typically associated with the ancestral form of cisco (e.g., benthic habitats) and phenotypic diversity (blackfin and cisco) also continue to expand niche size in Mysis-based food webs. Partial ecological speciation based on a large niche space appears to be present in one lake (Cauchon Lake) where use of alternative habitats is the only real difference in cisco. The presence of blackfin expands niche space in Cedar and Radiant Lakes. This was not matched in Hogan Lake where niche space was relatively smaller with similar forms. Possible reasons for this discrepancy may be related to the asymmetric basin of Hogan Lake and whether the two forms overlap during cool and cold-water periods of the annual temperature cycle. By comparing trophic niche size among lakes with and without Mysis, we conclude that Mysis provides a key ecological opportunity for cisco diversity in our study lakes and likely more widely.

Keywords: cisco diversity; ecological speciation; isotopes.

FIGURE 1

The native distribution of cisco (Coregonus artedi sensu lato) in a digital elevation map of Algonquin Park. Light areas are relatively high elevation and dark areas are lower elevation. The park boundary is in black (solid line). The extent of Mysis diluviana presence is defined by the red line matching an elevation of 381 m. The lower elevation area bounded by the 381 m contour is the area of inundation by Lake Algonquin drainage beginning in the west at the town of Fossmill and flowing east. Cisco colonized lakes within the drainage system and lakes above the 381 m contour. Study lakes are numbered as: (1) Manitou; (2) Three Mile; (3) Mink; (4) Cauchon; (5) Carl Wilson; (6) Cedar; (7) Catfish; (8) Hogan; (9) Radiant; (10) Grand

FIGURE 2

The Standard Elipse Area (SEA_B; o/oo²) of cisco (C. artedi sensu lato) for the lake set. The dominant diel migrator is either Chaoborus (=C) or Mysis (=M) among the lake set. Single phenotype refers to cisco that are the typical form for the species including pelagic only habitat occupancy, gill raker distribution, and size. Pelagic and benthic habitat refers to cisco captured in bottom set gillnets and pelagic gillnets. Phenotypic diversity refers to either two forms (blackfin and cisco forms; Cedar and Hogan Lakes) or blackfin form primarily with relatively few cisco forms (Radiant Lake). Box plots of posterior estimates for SEA_B are presented with the median (point), 50% credible intervals (smallest box), 75% credible intervals, and 95% credible intervals (largest box)

FIGURE 3

Plausible isotopic niche space for ciscoes in lakes with both pelagic (blue) and benthic (black) captures of ciscoes. Axis values are δ¹⁵N and δ¹³C relative to baseline values for each lake (δ¹⁵N_fish − δ¹⁵N_base; δ¹³C_fish − δ¹³C_base). Note the scale of relative δ¹³C varies with each figure panel

FIGURE 4

PCA analysis of ciscoes in lakes where pelagic and benthic nets captured ciscoes. Phenotypic (size, shape, and gill raker number) and isotopic niche data are included in the PCA where all variables were recorded for each specimen. Morphological data based on digitized landmarks from Piette‐Lauzière et al. (2019). Insert is the pattern of variable loading of PC1 and 2 in the PCA and associated gray scale is the percent of variation accounted for in the loadings

FIGURE 5

Plausible isotopic niche space for blackfin and cisco forms in Cedar and Hogan Lakes based on bimodality in gill raker number (Bell et al., 2019). Axis values are δ¹⁵N and δ¹³C relative to baseline values for each lake (δ¹⁵N_fish − δ¹⁵N_base; δ¹³C_fish − δ¹³C_base). Note the scale of relative δ¹³C varies with each figure panel