Semidiurnal temperature changes caused by tidal front movements in the warm season in seabed habitats on the Georges Bank northern margin and their ecological implications

This article is distributed under the terms of the Creative Commons Public Domain. The definitive version was published in PLoS ONE 8 (2013): e55273, doi:10.1371/journal.pone.0055273.Georges Bank is a large, shallow feature separating the Gulf of Maine from the Atlantic Ocean. Previous studies demonstrated a strong tidal-mixing front during the warm season on the northern bank margin between thermally stratified water in the Gulf of Maine and mixed water on the bank. Tides transport warm water off the bank during flood tide and cool gulf water onto the bank during ebb tide. During 10 days in August 2009, we mapped frontal temperatures in five study areas along ~100 km of the bank margin. The seabed “frontal zone”, where temperature changed with frontal movment, experienced semidiurnal temperature maxima and minima. The tidal excursion of the frontal boundary between stratified and mixed water ranged 6 to 10 km. This “frontal boundary zone” was narrower than the frontal zone. Along transects perpendicular to the bank margin, seabed temperature change at individual sites ranged from 7.0°C in the frontal zone to 0.0°C in mixed bank water. At time series in frontal zone stations, changes during tidal cycles ranged from 1.2 to 6.1°C. The greatest rate of change (−2.48°C hr−1) occurred at mid-ebb. Geographic plots of seabed temperature change allowed the mapping of up to 8 subareas in each study area. The magnitude of temperature change in a subarea depended on its location in the frontal zone. Frontal movement had the greatest effect on seabed temperature in the 40 to 80 m depth interval. Subareas experiencing maximum temperature change in the frontal zone were not in the frontal boundary zone, but rather several km gulfward (off-bank) of the frontal boundary zone. These results provide a new ecological framework for examining the effect of tidally-driven temperature variability on the distribution, food resources, and reproductive success of benthic invertebrate and demersal fish species living in tidal front habitats.This study was supported by salary funds from the regular annual salary budget from Northeast Fisheries Science Center (NEFSC) and United States Geological Survey Woods Hole Coastal and Marine Science Center (USGS WH C&MSC), respectively; ship time funds from the NEFSC annual budget for days-at-sea ship operations; equipment from the NEFSC and USGS WH C&MSC annual equipment budgets

Water depth, seabed temperature, and temperature change along transects across the tidal front.

<p>Temperature change from high to low tide is shown for the parts of transects where seabed temperature is influenced (Y) or not influenced (N) by frontal movement. Station pair groups (column 2) are numbered in each area from north to south (1–10; i.e. off-bank to on-bank). Each pair group contains up to 8 pairs of CTD stations, at which temperature was recorded at high or low tide. H Temp and L Temp are the ranges of seabed temperatures within the pair group measured at high and low tide, respectively. H-L ΔT is the range of temperature differences between high and low tides at stations within the pair group. This value is the criterion on which frontal influence is based. If the minimum value for H-L ΔT is >1.0°C, the pair group is considered influenced by frontal movement. Note that all station pairs in area D, Transect 23 were influenced. Area names (A-E) are as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-g001" target="_blank">Figure 1</a>; names of transects (TXX) across the tidal front are as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-g002" target="_blank">Figures 2</a>,<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-g003" target="_blank">3</a>,<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-g004" target="_blank">4</a>,<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-g005" target="_blank">5</a>,<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-g006" target="_blank">6</a>. See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-g007" target="_blank">Figure 7</a> for locations of individual stations along CTD transects. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone.0055273.s001" target="_blank">Table S1</a> includes data for all 60 station pairs.</p

Detail map of study area C showing subareas and seabed temperature stations.

<p>Data were obtained from the northern margin of Georges Bank in August 2009. Subareas 1–8 of are shown as boxes subdividing the larger area C box, labeled just inside their right (northeastern) margin. Dashed line is path of CTD transect 18 that was sampled around high and low tides (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-g007" target="_blank">Fig. 7C</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-t001" target="_blank">Table 1</a>). TS is the site of time-series station (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-g008" target="_blank">Fig. 8</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-t003" target="_blank">Table 3</a>). Other symbols and depth contours are as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-g002" target="_blank">Fig. 2</a>.</p

Detail map of study area B showing subareas and seabed temperature stations.

<p>Data were obtained from the northern margin of Georges Bank in August 2009. Subareas 1–7 are shown as boxes subdividing the larger area B box, labeled just inside their right (northeastern) margin. Dashed line is path of CTD transect 19 that was sampled around high and low tides (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-g007" target="_blank">Fig. 7B</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-t001" target="_blank">Table 1</a>). TS is the site of time-series station (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-g008" target="_blank">Fig. 8</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-t003" target="_blank">Table 3</a>). Other symbols and depth contours are as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-g002" target="_blank">Fig. 2</a>.</p

Map showing the location of Georges Bank (inset) and study areas.

<p>The locations of study areas A-E (boxes) and 464 seabed temperature stations (dots) occupied in the August 4–13, 2009 time period are indicated. Numbers 1–3 in rectangles mark tidal current prediction sites shown on NOAA Chart 13200 <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone.0055273-National2" target="_blank">[43]</a>. Numbers 1–4 in triangles mark locations of moored instruments of Loder et al. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone.0055273-Loder2" target="_blank">[18]</a> in July 1988. GS marks location of tidal height predictions on Georges Shoal <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone.0055273-National1" target="_blank">[42]</a>. Isobaths from NOAA Chart 13200 <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone.0055273-National2" target="_blank">[43]</a> are labeled in meters.</p

Detail map of study area E showing subareas and seabed temperature stations.

<p>Data were obtained from the northern margin of Georges Bank in August 2009. Subareas 1–6 are shown as boxes subdividing the larger area E box, labeled just inside their right (northeastern) margin. Dashed line is the path of CTD transect 24 that was sampled around high and low tides (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-g007" target="_blank">Fig. 7F</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-t001" target="_blank">Table 1</a>). TS is the site of time-series station (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-g008" target="_blank">Fig. 8</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-t003" target="_blank">Table 3</a>). Other symbols and depth contours are as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-g002" target="_blank">Fig. 2</a>. Data was insufficient to characterize the un-numbered subarea between subareas 3 and 4.</p

Detail map of study area D showing subareas and seabed temperature stations.

<p>Data were obtained from the northern margin of Georges Bank in August 2009. Subareas 1–8 are shown as boxes subdividing the larger area D box, labeled just inside their right (northeastern) margin. Dashed lines are paths of CTD transects 22 and 23 that were sampled around high and low tides (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-g007" target="_blank">Figs. 7b D and E</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-t001" target="_blank">Table 1</a>). TS is the site of time-series station (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-g008" target="_blank">Fig. 8</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-t003" target="_blank">Table 3</a>). Other symbols and depth contours are as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-g002" target="_blank">Fig. 2</a>.</p

Hourly rates of seabed temperature change at the time-series site in study area A.

<p>Observations were made at intervals ranging from 0.93 to 1.32 hr over a 12.22-hr period. They extended from 0.43 hr before (−) high tide (Hi) to 0.50 hr before (−) high tide and encompassed both flood (F) and ebb (E) stages. Rates of temperature change (^oC hr⁻¹) were calculated using differences in seabed temperature (ΔT) and the time elapsed from the previous observation. Rates were highest at or just after mid-ebb tide (CTD sta 059, −2.48^oC hr⁻¹; CTD sta 060, −1.83^oC hr⁻¹) and just before mid-flood tide (CTD sta 070, 1.14^oC hr⁻¹). Maximum flood and ebb rates of temperature change used for Area A in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-t003" target="_blank">Table 3</a> are shown in bold type here. No data were collected at the exact times of high or mid-flood tides, although both were bracketed. Low rates of temperature change bracketed low tide (slack water; CTD sta 061, 066, 069). Temperature decreased during ebb tide (CTD sta 057-061) and increased during flood tide (CTD sta 069-073). See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-g008" target="_blank">Figure 8</a> for temperature cross-section at the site.</p

Detail map of study area A showing subareas and seabed temperature stations.

<p>Data was obtained from the northern margin of Georges Bank in August 2009. Subareas 1–7 of are shown as boxes subdividing the larger area A box, labeled just inside their right (eastern) margin. Dashed line is path of CTD transect 16 that was sampled around high and low tides (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-g007" target="_blank">Fig. 7A</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-t001" target="_blank">Table 1</a>). TS is the site of time-series station (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-g008" target="_blank">Fig. 8</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-t003" target="_blank">Tables 3</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-t004" target="_blank">4</a>). Hachures show distribution of temperature data used to characterize the effect of frontal movement in each subarea (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-g011" target="_blank">Fig. 11</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone-0055273-t005" target="_blank">Table 5</a>). Black squares are hydrocast temperature observations on transects and at time-series stations. Black triangles are temperature observations at the starts and ends of video drift stations. Isobaths from NOAA Chart 13200 <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055273#pone.0055273-National2" target="_blank">[43]</a> are labeled in meters.</p

Rapid temperature changes in CTD temperature data collected along Seaboss video drift tracks.

<p>Examples of rapid temperature changes within the tidal front in off-bank subareas. A. Single rapid temperature changes (up to 0.6°C); sta. 908047, 801 m drift. B. Multiple rapid (4–5 min duration) saw tooth fluctuations (up to 0.3°C); sta. 908024, 498 m drift. C. Multiple rapid (6 min duration) saw tooth temperature fluctuations (up to 2–3°C); sta. 908034, 812 m drift.</p