Alternative Methods for Studying Stratification Dynamics on Discrete and Continuous Time
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1 1 Running Head: Methods for studying stratification 2 3 Title: Alternative methods for studying stratification dynamics on discrete and continuous time 4 scales 5 6 Katherine Hudson, Northeastern University, Marine and Environmental Sciences, 430 Nahant 7 Road, Nahant, MA 01908 8 2 9 Abstract 10 Stratification is an important driver for many biological and ecological processes across 11 benthic and pelagic habitats in the world ocean. However, stratification dynamics are still 12 undersampled due to limitations of current methods. Current methodologies rely primarily on 13 CTD and Niskin bottle data to develop stratification profiles, that are then compared over time. 14 Here, we describe two new methodologies which utilize remote sensing technologies for 15 examining stratification dynamics on discrete and continuous time scales. The first, focusing on 16 thin layers and zooplankton distributions in the water column, utilizes a Remotely Operated 17 Vehicle (OpenROV version 2.8) to record vertical transects in discrete time using a low-power 18 lens placed periodically over an HD imager. The second utilizes a customizable mooring system 19 and thermistor strings to continuously observe stratification as well as dynamic phenomena such 20 as internal waves. Using these methods, physical phenomena such as internal waves and thin 21 layers were observed with the continuous and discrete methods, respectively. These 22 methodologies allow for the observation of stratification dynamics on a variety of time and 23 spatial scales. A model was constructed in R to examine the effects of perturbations of the 24 stratified layer on downwelling that could have consequences for deeper-water pelagic and 25 benthic organisms. Understanding stratification dynamics and their impacts on water column 26 biota and the benthos across temporal and spatial scales will become increasingly important as 27 climate change impacts the dynamics of the surface layer of the world ocean. 28 Key Words: stratification, dynamics, zooplankton population dynamics, remote sensing, 29 temporal scales, internal waves, thin layers 30 Introduction 3 31 The stratification of the water column, or the distribution of bodies of water according to 32 their relative densities, has been shown to impact physical and biological phenomena throughout 33 the world ocean (Li 2002; Leichter et al. 1996; Wang et al. 2007). Changes in stratification 34 dynamics have been shown to influence species distributions, drive physical events in the water 35 column, and even influence events such as hurricanes and tropical cyclones above the ocean 36 (Greer et al. 2014; Butman et al. 2006b; Kunze et al. 2002; Holligan et al. 1985) 37 Despite the importance of stratification dynamics to species distributions across the world 38 ocean, stratification dynamics remain poorly sampled (Eickstedt et al. 2007). Sampling of ocean 39 stratification primarily occurs with CTDs, a group of ocean instruments capable of measuring 40 conductivity, temperature, and depth (Thompson and Emery 2014). These instruments can be 41 used to construct discrete temperature, salinity, and density profiles as a function of depth 42 (Thompson and Emery 2014). Data from CTD casts have been used previously to construct 43 reliable, long-term time series datasets that describe the seasonal changes in water column 44 structure and stratification (Steinberg et al. 2001). These data have been extremely influential to 45 describing the ocean circulation system present throughout the world ocean (Steinberg et al. 46 2001). However, these measurements are discrete (Thomson and Emery 2014). As a result, the 47 data they can collect are ultimately limited by their sampling frequency (Thomson and Emery 48 2014). 49 For example, the Bermuda Institute of Ocean Sciences (formally the Bermuda Biological 50 Research Station) has been following this sampling regime since 1954 with the development of 51 the Bermuda Atlantic Time-Series (BATS) study (Steinberg et al. 2001). While the data 52 collected at BATS is extremely valuable and has resulted in a wide-range of publications, the 53 sampling frequency of approximately once a month limits the researchers and scientists from 4 54 drawing concrete conclusions on what occurs at the study locations, or extrapolating those 55 results, on small time scales (Doney et al. 1996; Thompson and Emery 2014). 56 Currently, there are very few methods available for collecting data on continuous time 57 scales. One of the most popular of these are temporary mooring systems that can be deployed 58 with instrumentation specific to the needs of the researcher and the questions at hand (Butman et 59 al. 2006a). Such mooring systems have been used to study physical and biological phenomena 60 such as internal waves in Stellwagen Bank and harmful algal blooms in the Gulf of Maine 61 (Butman et al. 2006a, K. Hudson, pers. obs.). Instrument platforms and underwater vehicles, 62 autonomous or otherwise, have also been deployed to collect continuous data on the world ocean 63 (Eriksen et al. 2001). However, these systems are often only deployed for a single season and are 64 difficult to recover in inclement conditions (Pillsbury et al. 1969). 65 Another significant limitation to current stratification sampling methods is the cost of 66 instrumentation and ship time (Eriksen et al. 2001). CTD instruments, often included with 67 sampling bottle arrays, cost thousands of dollars, depending on the depth rating of the instrument 68 (Thompson and Emery 2014). Instruments capable of taking continuous measurements range can 69 cost upwards of $5,000 (Pillsbury et al. 1969). Research cruises to collect these data and deploy 70 the necessary instruments also can cost as much as $25,000 per day at sea (K. Hudson, pers. 71 obs). The high costs of both instruments and ship time often make up a significant portion of 72 grant budgets. Therefore, there is a significant need to develop relatively low-cost 73 instrumentation that can produce high quality and reliable data. 74 This study aims to address this need for data to be produced on a continuous time scale 75 and be relatively low cost when compared to traditional methods. Using northern Massachusetts 76 Bay as a study site, moorings like those used to study internal waves off Stellwagen Bank were 5 77 constructed (Butman et al. 2006). These moorings included thermistor strings of Onset HOBO 78 temperature loggers, low-cost temperature loggers ranging between $50 - $200 per device. 79 Inspired by the Massachusetts Bay Internal Wave Experiment in 1998 and work by John Witman 80 in the Gulf of Maine, three moorings were deployed off Nahant, MA and Rockport, MA to 81 observe stratification dynamics, including internal wave phenomena, during the summer months 82 of 2016 (Butman et al. 2006; Witman et al. 1993; Witman et al. 2004). 83 Internal waves occur in stratified waters and propagate along the stratification boundary 84 (Haury et al. 1979). They are formed by a disturbance in this boundary layer, which is usually 85 created by the movement of water due to tides over a large geographic feature, such as a ridge or 86 seamount (Haury et al. 1979; Helfrich and Melville 2006). These phenomena, in addition to 87 other stratification processes, have been shown to have significant impacts on plankton 88 distributions throughout the water column and can induce downwelling events (Lai et al. 2010; 89 Scotti and Pineda 2007; Shanks 1983; Witman et al. 1993). 90 In addition, an open-source remotely operate vehicle (ROV) by OpenROV was used to 91 make visual observations of zooplankton. Using the HD imager aboard the ROV and a 92 magnifying lens placed in front of the lens, the OpenROV v. 2.8 was deployed to observe how 93 zooplankton are impacted by stratification dynamics. Although this is technically discrete 94 method of observing plankton dynamics in response to changes in stratification dynamics, it is 95 hypothesized that the collaboration of these technologies provides a more holistic view of 96 stratification dynamics and their overall ecological impacts in northern Massachusetts Bay. 97 Methods 98 Continuous Observations – Thermistor String Mooring System 6 99 To continuously measure stratification dynamics, vertical moorings were constructed in 100 the spring of 2016. Moorings were constructed to hold HOBO thermistor strings and allow for 101 simple and quick recovery, data download, and redeployment. Three moorings were constructed 102 to accommodate deployment water column depths of approximately 30, 80, and 50 m. The top 103 and bottom 6 m of these moorings consisted of ¾-inch braided line to accommodate added stress 104 from the buoys and anchors. One-ft diameter Norway buoys were used through the testing phase. 105 When deployed in Rockport, these buoys were paired with 10 m of line between them in the 106 event one buoy was lost. Anchors were constructed from recycled brake pads with the line 107 looped through the center of the brake pad and passing the other end of the line through the eye 108 splice. When deployed in Nahant, only one anchor was used and weighed approximately 60 lbs. 109 In Rockport, anchors were deployed in pairs and weighed a cumulative 80 lbs to reduce the risk 110 of dragging. The remaining line between the buoy and anchor was 1/4-inch braided line. Each 111 line had an eye splice at each end, which contained a metal thimble of the appropriate diameter. 112 Sections of the mooring system were attached using ½-inch shackles. The buoys were attached to 113 the gear using a whale-friendly break away swivel (weak link), that has a breaking strength of 114 600 lbs. 115 The thermistor string was constructed using woven strap material with a width of 1 inch. 116 The strap was cut to be equal to the length of the ¼-inch line section of the mooring. When the 117 length required exceeded the length of the strap available in a single unit, sections of strap were 118 connected with bowline knots.