skip to main content

Attention:

The NSF Public Access Repository (NSF-PAR) system and access will be unavailable from 5:00 PM ET until 11:00 PM ET on Friday, June 21 due to maintenance. We apologize for the inconvenience.


Title: Standing Crop, Turnover, and Production Dynamics of Macrocystis pyrifera and Understory Species Hedophyllum nigripes and Neoagarum fimbriatum in High Latitude Giant Kelp Forests

Production rates reported for canopy‐forming kelps have highlighted the potential contributions of these foundational macroalgal species to carbon cycling and sequestration on a globally relevant scale. Yet, the production dynamics of many kelp species remain poorly resolved. For example, productivity estimates for the widely distributed giant kelpMacrocystis pyriferaare based on a few studies from the center of this species' range. To address this geospatial bias, we surveyed giant kelp beds in their high latitude fringe habitat in southeast Alaska to quantify foliar standing crop, growth and loss rates, and productivity ofM. pyriferaand co‐occurring understory kelpsHedophyllum nigripesandNeoagarum fimbriatum. We found that giant kelp beds at the poleward edge of their range produce ~150 g C · m−2· year−1from a standing biomass that turns over an estimated 2.1 times per year, substantially lower rates than have been observed at lower latitudes. Although the productivity of high latitudeM. pyriferadwarfs production by associated understory kelps in both winter and summer seasons, phenological differences in growth and relative carbon and nitrogen content among the three kelp species suggests their complementary value as nutritional resources to consumers. This work represents the highest latitude consideration ofM. pyriferaforest production to date, providing a valuable quantification of kelp carbon cycling in this highly seasonal environment.

 
more » « less
Award ID(s):
1752600
NSF-PAR ID:
10385813
Author(s) / Creator(s):
 ;  
Publisher / Repository:
Wiley-Blackwell
Date Published:
Journal Name:
Journal of Phycology
Volume:
58
Issue:
6
ISSN:
0022-3646
Page Range / eLocation ID:
p. 773-788
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    Variability in primary producers' responses to environmental change may buffer higher trophic levels against shifts in basal resource composition. Then again, in instances where there is a lack of functional redundancy because consumers rely on a few species to meet their energetic requirements at specific times of the year, altered community production dynamics may significantly impact food web resilience. In high‐latitude kelp forests, a complementary annual phenology of seaweed production supports coastal marine consumers' metabolic needs across large seasonal variations in their environment. Yet, marine consumers in these systems may face significant metabolic stress under the pronounced low pH conditions expected in future winters, particularly if they lack the resources to support their increased energetic demands. In this study, we investigate how the growth and nutritional value of three dominant, coexisting macroalgal species found in subpolar kelp forests will respond to ocean acidification and warming in future winter and summer seasons. We find that the three kelpsMacrocystis pyrifera,Hedophyllum nigripes, andNeoagarum fimbriatumdiffer in their vulnerability to future environmental conditions, and that the seasonal environmental context of nutrient and light availability shapes these responses. Our results suggest that poleward fringe populations ofM. pyriferamay be relatively resilient to anticipated ocean warming and acidification. In contrast, ocean warming conditions caused a decrease in the biomass and nutritional quality of both understory kelps. Considering the unique production phenology ofH. nigripes, we emphasize that negative impacts on this species in future winters may be of consequence to consumer energetics in this system. This work highlights how interspecific variation in autotrophs' responses to global change can disrupt the diversity and phenological structure of energy supply available to higher trophic levels.

     
    more » « less
  2. Abstract

    Canopy‐forming kelps are foundational species in coastal ecosystems, fixing tremendous amounts of carbon, yet we know little about the ecological and physiological determinants of dissolved organic carbon (DOC) release by kelps. We examined DOC release by the bull kelp,Nereocystis luetkeana, in relation to carbon fixation, nutrient uptake, tissue nitrogen content, and light availability. DOC release was approximately 3.5 times greater during the day than at night. During the day,N. luetkeanablades released an average of 16.2% of fixed carbon as DOC. Carbon fixation increased with light availability but DOC release did not, leading to a lower proportion of fixed carbon released as DOC at high light levels. We found no relationship between carbon fixation and DOC release rates measured concurrently. Rather, DOC release byN. luetkeanablades declined with marginal significance as blade tissue nitrogen content increased and with experimental nitrate addition, supporting the role of stoichiometric relationships in DOC release. Using a stable isotope (13C) tracer method, we demonstrated that inorganic carbon is rapidly fixed and released byN. luetkeanablades as13DOC, within hours. However, recently fixed carbon (13DOC) comprised less than 20% of the total DOC released, indicating that isotope studies that rely on tracer production alone may underestimate total DOC release, as it is decoupled from recent kelp productivity. Comparing carbon and nitrogen assimilation dynamics of the annual kelpN. luetkeanawith the perennial kelpMacrocystis pyriferarevealed thatN. luetkeanahad significantly higher carbon fixation, DOC production and nitrogen uptake rates per unit dry mass. Both kelp species were able to perform light‐independent carbon fixation at night. Carbon fixation by the annual kelpN. luetkeanais as high as 2.35 kg C·m−2·yr−1, but an average of 16% of this carbon (376 g C·m−2·yr−1) is released as DOC. As kelp forests are increasingly viewed as vehicles for carbon sequestration, it is important to consider the fate of this substantial quantity of DOC released by canopy‐forming kelps.

     
    more » « less
  3. Abstract

    Urea is an available and readily used source of nitrogen for giant kelp,Macrocystis pyrifera, but little is known about its potential importance for sustaining growth. Results of kinetic experiments indicate urea uptake saturates at an average maximum rate (Vmax) of 0.73–0.92 μmol N g dw−1h−1with a half saturation constant (Ks) of 1.02–1.08 μM. The affinity of giant kelp for urea was high relative to that reported for other seaweeds. However, results of similar kinetics experiments with natural, co‐occurring phytoplankton communities indicate that the rate of urea uptake by phytoplankton was > 10‐fold higher than that of giant kelp. Urea uptake by giant kelp decreased 3–12% in darkness (relative to in light) compared to a 66–85% decline for phytoplankton. Similar differences were observed for ammonium and nitrate, suggesting that light intensity and photocycles influence the outcome of competition for N between giant kelp and phytoplankton. Monthly measures of urease in kelp tissues revealed persistent activity at levels that were 100‐fold higher than rates of urea uptake (0.13–0.35 μmol N g fw−1min−1). This finding, coupled with unsuccessful efforts to induce additional urease activity through substrate additions, suggests that urease plays a role in giant kelp physiology beyond that of processing urea taken up from the environment. Collectively, our results suggest giant kelp uses multiple forms of N including urea to sustain year‐round growth. Its consistent capacity to acquire N during both day and night may help offset its low uptake rates relative to phytoplankton and increase its ability to compete for N during periods of low N availability.

     
    more » « less
  4. We conducted a population genetic analysis of the stalked kelp,Pterygophora californica, in the Santa Barbara Channel, California,USA. The results were compared with previous work on the genetic differentiation of giant kelp,Macrocystis pyrifera,in the same region. These two sympatric kelps not only share many life history and dispersal characteristics but also differ in that dislodgedP. californicadoes not produce floating rafts with buoyant fertile sporophytes, commonly observed forM. pyrifera. We used a comparative population genetic approach with these two species to test the hypothesis that the ability to produce floating rafts increases the genetic connectivity among kelp patches in the Santa Barbara Channel. We quantified the association of habitat continuity and oceanographic distance with the genetic differentiation observed in stalked kelp, like previously conducted for giant kelp. We compared both overall (across all patches) and pairwise (between patches) genetic differentiation. We found that oceanographic transit time, habitat continuity, and geographic distance were all associated with genetic connectivity inP. californica, supporting similar previous findings forM. pyrifera. Controlling for differences in heterozygosity between kelp species using Jost'sDEST, we showed that global differentiation and pairwise differentiation were similar among patches between the two kelp species, indicating that they have similar dispersal capabilities despite their differences in rafting ability. These results suggest that rafting sporophytes do not play a significant role in effective dispersal ofM. pyriferaat ecologically relevant spatial and temporal scales.

     
    more » « less
  5. Kelp beds provide significant ecosystem services and socioeconomic benefits globally, and prominently in coastal zones of the California Current. Their distributions and abundance, however, vary greatly over space and time. Here, we describe long-term patterns of Giant Kelp (Macrocystis pyrifera) sea surface canopy area off the coast of San Diego County from 1983 through 2019 along with recent patterns of water column nitrate (NO3-) exposure inferred fromin situtemperature data in 2014 and 2015 at sites spanning 30 km of the coastline near San Diego California, USA. Site-specific patterns of kelp persistence and resilience were associated with ocean and climate dynamics, with total sea surface kelp canopy area varying approximately 33-fold over the almost 4 decades (min 0.34 km2in 1984; max 11.25 km2in 2008, median 4.79 km2). Site-normalized canopy areas showed that recent kelp persistence since 2014 was greater at Point Loma and La Jolla, the largest kelp beds off California, than at the much smaller kelp bed off Cardiff. NO3-exposure was estimated from an 11-month time series ofin situwater column temperature collected in 2014 and 2015 at 4 kelp beds, using a relationship between temperature and NO3-concentration previously established for the region. The vertical position of the 14.5°C isotherm, an indicator of the main thermocline and nutricline, varied across the entire water column at semidiurnal to seasonal frequencies. We use a novel means of quantifying estimated water column NO3-exposure integrated through time (mol-days m-2) adapted from degree days approaches commonly used to characterize thermal exposures. Water column integrated NO3-exposure binned by quarters of the time series showed strong seasonal differences with highest exposure in Mar - May 2015, lowest exposure in Sep - Dec 2014, with consistently highest exposure off Point Loma. The water column integrated NO3-signal was filtered to provide estimates of the contribution to total nitrate exposure from high frequency variability (ƒ >= 1 cycle 30 hr-1) associated predominantly with internal waves, and low frequency variability driven predominantly by seasonal upwelling. While seasonal upwelling accounted for > 90% of NO3-exposure across the full year, during warm periods when seasonal upwelling was reduced or absent and NO3-exposure was low overall, the proportion due to internal waves increased markedly to 84 to 100% of the site-specific total exposure. The high frequency variability associated with internal waves may supply critical nutrient availability during anomalously warm periods. Overall, these analyses support a hypothesis that differences in NO3-exposure among sites due to seasonal upwelling and higher frequency internal wave forcing contribute to spatial patterns in Giant Kelp persistence in southern California. The study period includes anomalously warm surface conditions and the marine heatwave associated with the “Pacific Warm Blob” superimposed on the seasonal thermal signal and corresponding to the onset of a multi-year decline in kelp canopy area and marked differences in kelp persistence among sites. Our analysis suggests that, particularly during periods of warm surface conditions, variation in NO3-exposure associated with processes occurring at higher frequencies, including internal waves can be a significant source of NO3-exposure to kelp beds in this region. The patterns described here also offer a view of the potential roles of seasonal and higher frequency nutrient dynamics for Giant Kelp persistence in southern California under continuing ocean surface warming and increasing frequency and intensity of marine heatwaves.

     
    more » « less