Sea lamprey nests promote the diversity of benthic macroinvertebrate assemblages

Abstract

The habitat heterogeneity hypothesis states that increased habitat heterogeneity promotes species diversity through increased availability of ecological niches. We aimed at describing the local-scale (i.e. nest and adjacent substrate) effects of nests of the sea lamprey (Petromyzon marinus L.) as ecosystem engineer on macroinvertebrate assemblages. We hypothesized that increased streambed physical heterogeneity caused by sea lamprey spawning would modify invertebrate assemblages and specific biologic traits and promote reach-scale diversity. We sampled thirty lamprey nests of the Nive River, a river of the south western France with a length of 79.3 km and tributary of the Adour River, in three zones: the unmodified riverbed (upstream) and zones corresponding to the nest: the area excavated (pit) and the downstream accumulation of pebbles and cobbles (mound). The increased habitat heterogeneity created by lamprey was accompanied by biological heterogeneity with a reduced density of invertebrates (3777 ± 1332 individuals per m2 in upstream, 2649 ± 1386 individuals per m2 in pit and 3833 ± 1052 individuals per m2 in mound) and number of taxa (23.5 ± 3.9 taxa for upstream, 18.6 ± 3.9 taxa in pit and 21.2 ± 4.5 taxa for mound) in the pit compared to other zones. However the overall taxa diversity in nest increased with 82 ± 14 taxa compared to the 69 ± 8 taxa estimated in upstream zone. Diversity indices were consistent with the previous results indicating a loss of α diversity in pit but a higher β diversity between a pit and a mound than between two upstream zones, especially considering Morisita index accounting for taxa abundance. Trait analysis showed high functional diversity within zones with a reduced proportion of collectors, scrapers, shredders, litter/mud preference and small invertebrates in mound, while the proportion of "slabs, blocks, stones and pebbles" preference and largest invertebrates increased. Pit presented the opposite trend, while upstream had globally intermediate trait proportions. Our results highlight important effects on species and functional diversity due to habitat heterogeneity created by a nest-building species, what can ultimately influence food webs and nutrient processes in river ecosystems.

Fig 1. Photo of two sea lamprey nests in the Nive River.

The mound of cobbles, immediately downstream of the pit, is clearly visible. ©INRAE-GLISE.

Fig 2. Location of the nests sampled in the selected section of the Niver river in Saint-Martin-d’Arrossa, bypassed by a hydropower scheme.

Fig 3. Relationship between depth and current velocity for each nest at each zone.

Ellipses correspond to multivariate t-distribution.

Fig 4. Density of macroinvertebrates (ind/m²) (A) and number of taxa per sample (B) for the three zones studied.

With ns P > 0.05, *P ≤ 0.05, **P ≤ 0.01, ****P ≤ 0.0001.

Fig 5. Venn diagram with the distribution of taxa among zones.

The area of each part is proportional to the number of taxa indicated in absolute number and percentage of total taxa richness.

Fig 6. Log response ratios of Shannon and Pielou α diversity indices for each pit and each mound compared to the mean of Shannon and Pielou indices obtained for the upstream zones.

Dots correspond to the mean and error bars correspond to the standard deviation. When the LRR standard deviation does not cross the value “0”, it means that the effect is significantly positive (above 0) or negative (below 0).

Fig 7. Log response ratios of β diversity indices of all mound and pit pairwise indices, each compared to a pairwise upstream index among all possible upstream pairwise comparisons (upstream zone randomly selected, not directly upstream from the pit and mound considered).

Dots correspond to the mean and error bars correspond to the standard deviation. When the LRR standard deviation does not cross the value “0”, it means that the effect is significantly positive (above 0) or negative (below 0).

Fig 8. Median percentage of macroinvertebrate alimentation traits for the three zones studied.

Fig 9. Median percentage of macroinvertebrate substrate preferences for the three zones studied.

Fig 10. Median percentage of macroinvertebrate size traits for the three zones studied.

Fig 11. Redundancy analysis between nest characteristics and macroinvertebrate traits.

Slabs = slabs, blocks, stones and pebbles.