Long-term changes in kelp forests in an inner basin of the Salish Sea

Abstract

21Understanding the historical extent of biogenic habitats can provide insight into the nature 22 of human impacts and inform restoration and conservation actions. Kelp forests form an 23 important biogenic habitat that responds to natural and human drivers. Global concerns exist 24 about threats to kelp forests, yet long term information is limited and research suggests that 25 trends are geographically distinct. We examined distribution of the bull kelp Nereocystis 26 luetkeana over 145 years in South Puget Sound (SPS), a semi-protected inner basin in a fjord 27 estuary complex in the northeast Pacific Ocean. We synthesized 48 historical and modern 28 Nereocystis surveys and examined presence/absence within 1-km shoreline segments along 452 29 km of shoreline. Over the last 145 years, Nereocystis has been documented in 26% of the 30 shoreline segments. Its extent decreased 62% basin-wide between the 1870s and 2017, with 31 extreme losses in the two out of three sub-basins (96% in Central and 83% in West). In recent 32 years, almost all Nereocystis occurred in the East sub-basin. In the majority of segments where 33 Nereocystis disappeared, the most recent observation was 4 decades ago, or earlier. Multiple 34 natural and human factors that are known to impact kelp could have contributed to observed 35 patterns, but limited data exist at the spatial and temporal scale of this study. In some areas, 36 recent environmental conditions approached thresholds associated with decreased kelp 37 performance. Longstanding Nereocystis losses occurred exclusively in areas with relatively low 38 current velocities. Remaining Nereocystis predominantly occurred in areas where circulation is 39 stronger. Exceptions to this pattern demonstrate that additional factors outside the scope of this 40 study contributed to trajectories of Nereocystis persistence or loss. 42 Humans have altered coastal ecosystems for centuries, yet we frequently lack long-term 43 datasets to define a baseline that precedes significant human impacts and to identify changes 44 from that baseline. The need for long-term reference points was initially identified in the context 45 of global fisheries [1], but it is equally important to understand changes in biogenic habitats as 46 losses can trigger changes to broader ecosystem structure and services. Historical maps have 47 been used to estimate changes in the spatial extent of coastal habitats such as salt marshes, oyster 48 reefs, coral reefs and kelp forests over century time scales [2][3][4][5]. Extended temporal baselines 49 can enhance understanding of habitat variability and change. For example, historical nautical 50 charts were used to spatially and temporally extend baseline information for coral reef extent in 51 the Florida Keys, revealing greater loss than was previously quantified, along with newly 52 identified areas of intact offshore reefs [4]. Enhanced understanding of the magnitude and spatial 53 patterns of change in biogenic habitats can guide restoration and conservation...

Toward a conceptual framework for managing and conserving marine habitats: A case study of kelp forests in the Salish Sea

“…The other used long‐term data on N . luetkeana canopy extent and water column nitrogen concentrations in the South Puget Sound region to reveal a robust positive relationship between nutrient levels and canopy persistence (Berry et al, 2021) (Figure 5).…”

“…), herring ( Clupea pallasii ), and abalone ( Haliotis kamtschatkana ) (NMFS, 2005, 2014). Recently quantified declines in the extent of kelp forests in Puget Sound raised concerns regarding the availability of critical habitat for these threatened species which motivated the creation of the Puget Sound Kelp Conservation and Recovery Plan (Berry et al, 2021; Calloway et al, 2020). Although the drivers of the declines remain unclear, they are likely the result of cumulative effects from multiple natural and human stressors on the system such as increasing sea surface temperatures and incidences of marine heatwaves (Iwabuchi & Gosselin, 2019; Masson & Cummins, 2007), changes to watersheds and nearshore terrestrial environments (Hansen et al, 2013), and changes to marine ecological communities (Pietsch & Orr, 2015; Zier & Gaydos, 2016).…”

The role of environmental conditions in regulating long-term dynamics of an invasive seaweed

“…Fluctuations of abiotic conditions, such as temperature, waves, currents and food availability, influence the abundance and distribution of marine species, including invasive ones (Iles et al 2012;Bell et al 2015;Bertocci et al 2015;Chefaoui et al 2019;Burrows et al 2020;Rilov et al 2020;Berry et al 2021). As the oceans are rapidly changing due to anthropogenic activities, monitoring species abundance along with abiotic variables is essential to predict shifts in species distributions and their ecological effects on ecosystem structure and functioning (Bates et al 2018;Miloslavich et al 2018).…”

“…Experimental studies are, however, warranted to assess the thermal physiological thresholds for this species in the introduced range and to improve predictions of its future spread. Seasonal and interannual fluctuations in resource availability and hydrodynamic conditions can also play a role in driving seaweed abundance and distribution dynamics (Cavanaugh et al 2011;Bell et al 2015; Berry et al 2021). For instance, along exposed coastlines, physical disturbance due to severe winter storms has been shown to be a major determinant of kelp cover.…”

Automated satellite remote sensing of giant kelp at the Falkland Islands (Islas Malvinas)

“…Marine heatwave events (MHW) -high temperature episodes that coincide with lower nutrient availability and can facilitate the spreading of disease and non-native species [47] -have often preceded regional declines in canopy-forming kelp [44,45,[48][49][50]. We did not detect a sustained trend or large annual anomalies in satellite estimates of nitrate or temperature during the period spanning 2002 to 2021 (SST data from the MODIS Aqua satellite began in 2002), which is consistent with our time-series analysis that did not detect changes in giant kelp canopy area.…”