MECHANISMS DRIVING KELP DETRITAL PARTICULATE ORGANIC CARBON (POC) UNCERTAINTIES AND POPULATION DYNAMICS IN THE EASTERN SHORE ISLANDS, NOVA SCOTIA

Abstract

Kelps are large brown algae that sustain marine life, support fisheries, and provide valuable ecosystem services. Carbon cycling is an important ecosystem service for kelp forests as they are among the most productive and widespread coastal vegetated ecosystems globally. However, current detrital carbon production estimates from kelp forests carry large uncertainties and studies rarely offer tangible solutions to improve them. We addressed this gap by quantifying population dynamics and demographic rates for the two dominant kelp species in Nova Scotia, Laminaria digitata and Saccharina latissima and estimate the detrital carbon production of both species. These estimates account for kelp size, since larger individuals produce more detritus, and for seasonal patterns, as most detrital production occurs in autumn. We found that most of the uncertainty in detrital carbon production estimates stems from variability in kelp density and population size structure. Incorporating seasonally varying erosion rates also reduces uncertainty as annual averages tend to over-simplify temporal patterns. Carbon cycling in kelp forests may be at risk as populations in Nova Scotia have undergone cycles of decline and recovery, with recent decades marked by substantial losses. While the causes of decline, warming waters, increased storm frequency, grazing by mesograzers, and invasive species, are known to affect individual kelps, their effects on population-level processes remain less understood. We constructed matrix population models to investigate the life history mechanisms driving the population dynamics of both species. We found that small individuals are critical for long-term population persistence. L. digitata depends on the year-round survival of small individuals, while S. latissima relies on autumn spore release, winter recruitment, and spring growth of small individuals. Stressors disrupting these life history events such as warming, turf algal competition, impacts from the invasive bryozoan Membranipora membranacea, and the mesograzer snail Lacuna vincta are therefore likely to drive kelp population dynamics. We also found that large individuals are essential for short-term population persistence and recovery. Both warming and M. membranacea negatively affect juvenile life histories and reduce the presence of large adults. The loss of large adults also would negatively impact detrital carbon production from kelps. These stressors are synergistic as warming promotes the population growth of M. membranacea. Together, our findings provide a mechanistic understanding for how continued ocean warming may undermine both the ecosystem functions and population resilience of kelp forests in Nova Scotia.