Date of Award
8-22-2022
Document Type
Open Access Thesis
Degree Name
Master of Science (MS)
Department
Biology
First Advisor
Karen Wilson
Second Advisor
Chris Maher
Third Advisor
Rachel Lasley-Rasher
Abstract
Oiadromous fish provide ecological subsidies to freshwater and marine food
webs, connecting both ecosystems. A main goal of the Penobscot River Restoration
Project was to increase connectivity between food webs by removing two mainstem
dams, improving fish passage, and reintroducing river herring through stocking.
Oiadromous fish now reach historic spawning habitat that was not accessible for
centuries. As a result, river herring runs in the Penobscot River increased from
2,336 fish in 2009 to over 3 million fish by 2018. To assess food web connectivity in
the Penobscot watershed, I analyzed stable isotopes from samples collected before
(2009-2010) and after (2020-2021) dam removals by sampling species ranging in
trophic level from piscivorous fish to baseline primary consumers from three
mainstem and three major upstream tributary sites. I targeted top fish predators
that can consume adult river herring directly. Pre-restoration, I found little evidence
of marine derived nutrient (MON) assimilation in freshwater food webs, with the
exception of a mainstem site below all dams. Post-restoration, MON assimilation
increased only below what is now the lowest dam on the river, likely due to
migration delays aggregating more fish for a longer period of time than in free-flowing
river sections. Where changes in MON assimilation occurred, I saw evidence
of bottom-up enrichment of the food web. This pattern of enrichment has been
measured in smaller rivers with spawning runs dominated by river herring. These
results may be one of the first in a river of this size (watershed area 22,300 km2)
and restoration of this magnitude, suggesting that even in larger rivers with greater
"dilution effects," effects of river herring on the transfer of nutrients from marine to
freshwaters are detectable. In the Penobscot Watershed, river herring currently
dominates the sea-run fish population but only comprise 20% of conservative
estimates of historic run size based on spawning habitat available before dam
construction. As sea-run species increase in abundance, I expect MDN to be
detectable beyond points of aggregation.
Recommended Citation
Brewer, Matthew MS, "Assessing Changes in Freshwater and Marine Food Web Connections Following Restoration on the Penobscot River, Maine, Using Stable Isotope Analysis" (2022). Student Scholarship. 12.
https://digitalcommons.usm.maine.edu/bio-students/12
Comments
Fourth Advisor: Graham Sherwood