Nutrient availability and senescence spatially structure the dynamics of a foundation species

Abstract

Disentangling the roles of the external environment and internal biotic drivers of plant population dynamics is challenging due to the absence of relevant physiological and abundance information over appropriate space and time scales. Remote observations of giant kelp biomass and photosynthetic pigment concentrations are used to show that spatiotemporal patterns of physiological condition, and thus growth and production, are regulated by different processes depending on the scale of observation. Nutrient supply was linked to regional scale (>1 km) physiological condition dynamics, and kelp forest stands were more persistent where nutrient levels were consistently high. However, on local scales (<1 km), internal senescence processes related to canopy age demographics determined patterns of biomass loss across individual kelp forests despite uniform nutrient conditions. Repeat measurements of physiology over continuous spatial fields can provide insights into complex dynamics that are unexplained by the environmental drivers thought to regulate abundance. Emerging remote sensing technologies that provide simultaneous estimates of abundance and physiology can quantify the roles of environmental change and demographics governing plant population dynamics for a wide range of aquatic and terrestrial ecosystems.

Disturbance and nutrients synchronise kelp forests across scales through interacting Moran effects

“…Further study may clarify the role of photoperiod in structuring synchrony across multiple processes (e.g. reproduction, recruitment, growth) that contribute to annual fluctuations in kelp canopy biomass, and how these differ geographically (Bell & Siegel, 2022). Additional research may also reveal how the global generalisability of our results is mediated by giant kelp phenotypic plasticity to diverse oceanographic conditions (Demes et al, 2009).…”

“…And how can interactions be synergistic on some timescales and simultaneously antagonistic on others, as we found for giant kelp forests in central California? First, note that large waves have immediate negative effects on kelp biomass, whereas elevated nutrients have positive effects that are delayed by one quarter (Appendix S2) because it takes several weeks to months for new kelp recruitment and growth to reach the water surface and achieve densities detectable from satellites (Bell & Siegel, 2022; Schiel & Foster, 2015). In central California, waves tend to achieve their annual maximum in the winter, whereas nutrients achieve their annual maximum in spring (Bell, Cavanaugh, Reed, & Siegel, 2015).…”

“…At scales of hundreds of metres, synchronous recruitment after mortality events can produce cohorts with similar age structure (Dayton et al, 1992). Because adult giant kelp sporophytes typically live for 2–3 years (Reed et al, 2008; Rosenthal et al, 1974), the growth and senescence of these cohorts may cause short‐interannual synchrony of kelp canopy biomass in areas not exposed to annual wave disturbance (Bell & Siegel, 2022; Rodriguez et al, 2013). On the other hand, the synchronising effect of dispersal may be interrupted by local processes that inhibit recruitment, such as grazing (Dean et al, 1984; Leonard, 1994), competition (Beckley & Edwards, 2021; Reed & Foster, 1984), or nutrient limitation (Deysher & Dean, 1986; Hernández‐Carmona et al, 2001).…”

Aquatic ecosystems science using an imaging spectrometer

“…36 Physiological condition is estimated through the measurement of the chlorophyll to carbon ratio (Chl:C) and is linked to changes in the external light and temperature/nutrient environment 37 and declines predictably with age. 38 The combination of Chl:C and biomass estimates will also be key for the assessment of net primary production.…”

Satellite Imagery-Estimated Intertidal Seaweed Biomass Using UAV as an Intermediary

“…Remote sensing observations offer several advantages over in situ surveys, such as a broad observation range, time and cost-effectiveness. The satellite remote sensing method is especially useful for obtaining a large amount of datasets over long time spans across large areas, with applications in monitoring the vegetation, including forests [13], grasslands [14] and giant kelp habitats [15]. These features share a high degree of distribution and homogeneity characteristics.…”