Regional Studies in Marine Science Influence of Internal Bores on Larval
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Regional Studies in Marine Science 20 (2018) 1–12 Contents lists available at ScienceDirect Regional Studies in Marine Science journal homepage: www.elsevier.com/locate/rsma Influence of internal bores on larval fish abundance and community composition Patrick J. Phelan a,*, John Steinbeck a, Ryan K. Walter b a Tenera Environmental Inc., San Luis Obispo, CA, USA b Physics Department, California Polytechnic State University, San Luis Obispo, CA, USA h i g h l i g h t s • A persistent, semi-diurnal internal tide occurs at the head of a submarine canyon. • The internal tide drives cold subthermocline waters (bores) onto the adjacent shelf. • Larval fish samples and bore activity are analyzed over a full year. • Along with seasonality internal bores are a key control on the larval fish community. article info a b s t r a c t Article history: A persistent semidiurnal internal tidal bore feature occurs at the head of the Monterey Bay Submarine Received 5 September 2017 Canyon and drives regular intrusions of cold, subthermocline waters onto the adjacent shelf. In this study, Received in revised form 19 January 2018 we examine the influence of this internal tidal bore feature on the larval fish community using over a Accepted 23 March 2018 year of periodic larval fish samples collected coincidently with physical measurements. Larval samples Available online 31 March 2018 were categorized into one of two water mass periods: a ``warm period'' representative of shallow coastal Keywords: shelf waters and a ``cold period'' characteristics of colder waters present during internal bore forcing. Larval fish Using multivariate statistical methods, we show warm and cold periods, along with seasonality, are the Internal waves and bores primary drivers of larval fish community composition. A significantly different community composition Larval abundance was observed between warm and cold water mass periods. This difference was primarily due to decreased Submarine canyon abundance in most taxa during the cold periods, and did not indicate an obvious shift in the assemblage Subthermocline water of the taxa. However, our data do indicate that some taxa may show higher abundance during cold periods compared to warm periods, but further studies are warranted. Along with seasonality, the presence/absence of subthermocline waters driven by internal bores appears to be a key control on nearshore larval fish community composition at this location. ' 2018 Elsevier B.V. All rights reserved. 1. Introduction even these larvae are still dependent on interactions with their en- vironment (Morgan et al., 2009; Shanks, 1983; Pineda et al., 2007). Early stage pelagic marine fish larvae and eggs are generally The distribution and abundance of adult populations can also be assumed to be limited in their ability to affect their location in affected by transport mechanisms that regulate the supply of ju- the marine environment (Moser and Watson, 2006) and so, like venile recruits (Morgan et al., 2011; Raimondi, 1991; Sponaugle many weak swimming larval forms, ocean currents are presumed and Cowen, 1996). Describing processes that affect larval transport to be the dominant driver of transport and subsequent patterns and abundance is therefore important in understanding commu- of distribution (Chia et al., 1984; Bradbury and Snelgrove, 2001). nity structure and population dynamics, and subsequently has a Behavior of late stage planktonic larvae may have a significant bearing on the management of marine resources such as fishery bearing on their distribution and abundance in the water column stock assessment, marine protected area design and management, (Shanks, 2009; Shanks et al., 2003; Leis, 2006; Cowen and Sponau- and the strategic planning of ocean intake locations (Fogarty and gle, 2009; Pineda et al., 2007) and so distribution may be partially Botsford, 2007; Hare and Walsh, 2007). uncoupled from transport mechanisms for these larvae; however, Larval dispersal and abundance are influenced by physical pro- cesses that act across a broad range of spatial and temporal scales * Corresponding author. (Gawarkiewicz et al., 2007; Bradbury and Snelgrove, 2001; Pineda E-mail address: [email protected] (P.J. Phelan). et al., 2007). In eastern boundary current upwelling systems, such https://doi.org/10.1016/j.rsma.2018.03.010 2352-4855/' 2018 Elsevier B.V. All rights reserved. 2 P.J. Phelan et al. / Regional Studies in Marine Science 20 (2018) 1–12 as the California Current, equatorward winds drive regional coastal 2016). The data presented here were collected as part of a wider ob- upwelling. This seasonal process acts to drive deep, cold, and servational study aimed at characterizing the physical and biolog- nutrient-rich waters close to the surface, thus fueling primary ical environment at the head of the Monterey Submarine Canyon productivity (cf. Walter et al., 2018). Coastal upwelling also has (MSC) (cf. Walter and Phelan, 2016). As part of this project, Walter the potential to generate and influence mesoscale features, such and Phelan(2016) documented the presence of semidiurnal tidal as ocean eddies, filaments, and persistent fronts (e.g., Harrison period pumping of cold-water masses from below the thermocline et al., 2013; Nidzieko and Largier, 2013; see Chao et al. 2017 (subthermocline waters) onto the adjacent shelf at this location. numerical model). At a regional scale, larval transport, abundance, These cold-water intrusions (internal bores) are the result of the and connectivity are most often associated with these mesoscale interaction of the internal tide with the canyon edge and are the features (Siegel et al., 2003; Mitarai et al., 2009; Bertness et al., dominant mode of physical variability at this site. The internal 1996; Kinlan and Gaines, 2003; Watson et al., 2011; Harrison et bores resulted in large changes in temperature throughout the al., 2013; Roughgarden et al., 1988). On a smaller-scale and closer year with alternating cold (onshelf flow from the canyon advecting to the shoreline, larval transport and abundance are thought to cold, subthermocline waters onto the shelf) and warm (offshelf be influenced by smaller-scale processes that drive cross-shelf flow back into the canyon returning shallow coastal shelf waters exchange (Pineda, 1994; Mullineaux and Mills, 1997; Cowen et al., to the shelf) periods at the study site. Furthermore, the internal 2007), and this may influence larval abundance and distribution bores displayed a distinct seasonality, with increased semidiurnal at these smaller spatial scales (Shanks, 1983; Shanks et al., 2014; temperature variance in the summer months during periods of Pineda, 1994; Vargas et al., 2004; Liévana McTavish et al., 2016; strong regional upwelling favorable winds and the subsequent Woodson et al., 2012; Nickols et al., 2013). shoaling of the offshore thermocline. In contrast, during the winter There is an established body of evidence demonstrating that months, the semidiurnal temperature variance decreased, with the supply of planktonic marine larvae to adult populations, in the occasional absence in bore activity, as the regional upwelling addition to other factors such as competition and predation that weakened and the offshore thermocline deepened. We refer the regulate adult population abundance and distribution patterns, reader to Walter and Phelan(2016) for a more detailed description. can play an important role in the ecology of marine populations We expand on this earlier work here and test the hypothesis that (e.g. Connell, 1985; Cowen, 1985; Gaines et al., 1985; Roughgarden internal bores influence the larval fish abundance and composition et al., 1988; Levin, 2006). For fishes with sedentary adult stages, at the head of the MSC. connectivity between patches of adults through larval dispersal is a major mechanism by which many marine taxa maintain popula- 2. Methods tion stability and persistence in the face of disturbance (Hasting and Botsford, 2006; Cowen and Sponaugle, 2009; Burgess et al., Monterey Bay is a large, semi-enclosed coastal embayment 2014), and factors such as physical processes that disrupt this featuring one of the largest submarine canyons on the west coast connectivity can have important effects on patterns of adult dis- of the United States (MSC – Fig. 1a). This biologically diverse marine tribution (Woodson et al., 2012). Indeed, many physical processes ecosystem is part of the Monterey Bay National Marine Sanctuary play an important role in determining patterns of larval abundance and features large commercial fisheries and some of the world's and ultimately the supply of settlement-competent larvae to adult largest giant kelp forests (Macroscystis pyrifera). The intense pro- habitats (Gaylord and Gaines, 2000; Roughgarden et al., 1988). A ductivity of Monterey Bay is partially driven by seasonal wind- better understanding of these processes will inform settlement driven coastal upwelling (Pennington and Chavez, 2000). Other and recruitment dynamics, as well as the distribution of adult physical conditions across the bay include a mixed semidiurnal populations (Gaines et al., 1985; Gaines and Roughgarden, 1987). surface tide, persistent upwelling jets and fronts, large ampli- This contribution examines the influence of a persistent, semid- tude internal waves, and nearshore internal bores (cf. Shea and iurnal internal bore feature on the local abundance of marine larval Broenkow, 1982; Breaker and Broenkow, 1994; Walter et al., 2012, fish over a complete annual