Wading into intertidal phenology: Establishing a coastal monitoring program

Climate-related impacts on terrestrial ecosystems are more extensively documented than impacts on marine ecosystems and coastal processes (Rosenzweig et al. 2008), yet the Gulf of Maine is warming faster than 99.85% of the global ocean (Pershing et al., 2015). Increasing sea surface temperatures influence the severity and frequency of environmental disturbances (e.g., Steiner et al. 2015) and change the species composition of marine ecosystems (Tan and Beal 2015; Kleisner et al. 2017, Sorte and White 2013). Signs of the Seasons (SOS) has developed a coastal phenology monitoring effort to help document biological responses to these changes, with an initial focus on rockweed.

Rockweed reproductive receptacles
Rockweed reproductive receptacles

Asco nodosum is a long-lived, commercially-harvested brown macroalgal species in the North Atlantic that is known as a “foundational species” because it forms underwater “forests” which shelter other species and influence ecosystem processes. It is an ideal indicator species for SOS phenology monitoring because it is easily identified, accessible from mid to low tide, and ubiquitous along rocky shorelines from US Mid-Atlantic states to the coast of Greenland. From a biological standpoint, rockweed is also ideal because it’s reproductive cycle is tied to water temperatures. The onset A. nodosum reproductive phases begins as water temperatures warm in spring and when Easily recognized reproductive receptacles enlarge, change color, and start to release gametes (eggs and sperm) when local sea surface temperatures reach their reproductive threshold. Gamete release begins at 6˚C (43˚F), and peaks at 10˚C (50˚F) (Bacon and Vadas, 1991). A long term record of the of the onset of A. nodosum reproductive phases will help researchers understand how shifts in ocean temperature influence its reproduction, and it has the potential for broader effects in nearshore marine ecosystems.

Guided by Dr. Muhlin, SOS developed three protocols for tracking A. nodosum phenology, growth rates, and associated water quality parameters, and trains volunteers on their use. The phenology and growth protocols are based on existing National Science Foundation protocols, and are used to monitor and document A. nodosum reproductive phenology throughout the safely-accessible monitoring season (February through November) and annually assesses age and growth rates of individual A. nodosum at each observation site. Water quality parameters (temperature and salinity) are also monitored throughout the season. At each site, SOS volunteers establish a 10-m transect parallel to the waterline which they monitor on a weekly to bi-monthly schedule. During each site visit, they record the presence of five distinguishable phenophases in each of 10 haphazardly chosen individual A. nodosum. Data are recorded in the field and subsequently submitted to the online SOS coastal monitoring database. Participants have submitted data from fifteen A. nodosum observing sites along the coast of Maine and New Hampshire since 2014.

In addition to providing data to understand the relationship between A. nodosum phenology and growth rates and nearshore water temperature and salinity, SOS data contribute to Dr. Muhlin’s broader research questions, which seek to understand how A. nodosum growth and reproduction influences nearshore marine ecosystems in a changing climate. The SOS data help to document variability in growth and phenological response across the 15 sites and among A. nodosum individuals, as well as interannual variability across all three monitoring protocols. The goal is to build a long-term data set that are needed to ascertain shifts in reproduction correlated with warming water temperatures. These long-term data sets will help to document biological responses to climate change in nearshore marine systems

Dr. Muhlin and her students are currently in their fourth year of an ongoing study that uses SOS data, information from intertidal temperature loggers, and microscopic analysis of gamete maturation, to refine the seminal A. nodosum reproduction model developed by Bacon and Vadas (1991), which first characterized temperature onset, midpoint, and termination of gamete release. Findings to date have been presented at state, regional, and international scientific conferences, and the study engages SOS volunteers in field research alongside Dr. Muhlin and her students.

By: Dr Jessica Muhlin. Edited by Beth Bisson, Esperanza Stancioff, and Sara Randall.


Bacon, L. and R.L. Vadas. 1991. A Model for gamete release in Ascophyllum nodosum (Phaeophyta). J. Phycol. 27: 166-173.

Kleisner, K.M.; Fogarty, M.J.; McGee, S.; Hare, J.A.; Moret, S.; Perretti, C.T; Saba, V.S.. Marine species distribution shifts on the U.S. Northeast Continental Shelf under continued ocean warming. Progress in Oceanography, 2017; 153: 24.

Pershing, A.; Alexander, M.; Hernandez, C.; Kerr, L.; Le Bris, A.; Mills, K.; Nye, J.; Record, N.; Scannell, H.; Scott, J.; Sherwood, G.; and Thomas, A. 2015. Slow adaptation in the face of rapid warming leads to collapse of the Gulf of Maine cod fishery. Science, 350 (6262), 809-812.

Rosenzweig C, Karoly D, Vicarelli M, Neofotis P, Wu Q, Casassa G, et al. 2008. Attributing physical and biological impacts to anthropogenic climate change. Nature, 453:353–358.

Sorte C.J.B, and J.W. White. 2013. Competitive and demographic leverage points of community shifts under climate warming. Proc R Soc B 280: 20130572.

Tan, E.B.P., Beal, B.F. 2015. Interactions between the invasive European green crab, Carcinus maenas (L.), and juveniles of the soft-shell clam, Mya arenaria L., in eastern Maine, USA. Journal of Experimental Marine Biology and Ecology, 462: 62-73.