Using high-resolution ocean timeseries data to givecontext to long term hydrographic sampling off Port

Hacking, NSW, AustraliaMoninya Roughan†, and Bradley D. Morris.

Dynamical Oceanography Laboratory, School of Mathematics and Statistics,University of New South Wales, Sydney NSW, 2052, Australia.

Sydney Institute of Marine Science, Mosman, NSW 2088†Email: mroughan@unsw.edu.au

Abstract—Through the development of the NSW node of theAustralian Integrated Marine Observing System (NSW-IMOS).A mooring array of 4 moorings has been developed off the coastof Sydney, Australia, providing more than 2 years of timeseriesdata on the Sydney shelf. Parameters measured include velocityand temperature, salinity, fluorescence, dissolved oxygen andturbidity.

This moored timeseries data complements the more than 70year time series (since 1942) of physical sampling at the PortHacking (Sydney, Australia) 50 m and 100 m sites, by providingspatial and temporal context. In this paper we investigate therelationship between the monthly vertical CTD profiles and thehigh temporal resolution moored timeseries, specifically the sub-monthly variability in the temperature and salinity observations.For the first time we have a timeseries of optical signals at twosites which can be used to give spatial and temporal contextto the monthly biogeochemical and phytoplankton record fromthe physical samples. We assess the significance of sub monthlyvariability relative to the annual signal. We also identify issueswith the optical signals obtained from the Wetlabs WQMS thatappear as a result of moving to an operational phase of theprogram where instruments are rotated frequently.

I. INTRODUCTION

The East Australian Current (EAC) flows southward alongthe east coast of New South Wales, transporting heat poleward.It drives upwelling in coastal waters [1], [2] and enhancesproductivity e.g. [3], [4] and biological connectivity, [5]. Inrecent years the number of oceanic observations along theNSW coastline have increased significantly with the instigationof the Australian Integrated Marine Observing System (IMOS,www.imos.org.au, [6], [7]). Numerical modelling efforts tounderstand the dynamics of the EAC e.g [8], [9] and thebiogeochemical response e.g. [10], [11] and subsequent con-nectivity [12] build on the long timeseries observations. OffPort Hacking, Sydney (34.05◦S) is one of Australia’s longestrunning hydrographic monitoring stations (since 1942), at the50 m and 100 m isobaths (PH050, PH100). The new mooredtimeseries data complements the long timeseries of physicalsampling, by providing spatial and temporal context. Withthe use of the timeseries data set we can deduce the activephysical forcing mechanisms (e.g. EAC Eddy encroachment),combined with the physical observations we can deduce the

biological response to the various forcing events. In this paperwe investigate the relationship between the monthly verticalCTD profiles and the moored timeseries, specifically the sub-monthly variability in the timeseries data. For the first timewe have a timeseries of optical signals at two sites which willbe used to give spatial and temporal context to the monthlybiogeochemical and phytoplankton record from the physicalsamples. We assess the significance of sub monthly variabilityrelative to the annual signal.

We also identify issues regarding data quality that appearas a result of moving to an operational phase of the programwhere instruments are rotated frequently.

We examine the temporal variability in temperature, salinityand fluorescence off the Sydney region, at meso-scale (sub-monthly) to seasonal timescales. Specifically:

• What is the significance of sub-monthly variation in tem-perature, salinity and fluorescence relative to the monthlyand annual signal.

• Is monthly physical sampling adequate to capture vari-ability in temperature, salinity and fluorescence?

II. METHODS

1) Biogeochemical Sampling: The CSIRO long term mon-itoring stations have been occupied nominally monthly since1942 (Figure 1). Instigated initially by CSIRO Marine Re-search (now CMAR), in recent decades the monthly mon-itoring has been run for CMAR by New South Wales Of-fice of Environment and Heritage. Initially temperature wasmonitored (using reversing thermometers) at PH050 (d=0,10,20,30,40,50 m) and PH100 (d=0,10,25,50,75,100 m). MonthlyCTD profiles have been taken at PH025, PH050, PH100and PH125 since 1997 (Table I). Zooplankton samples havealso been collected at the two stations since 1997. Monthlyplankton net samples from PH050 and PH100 have beencollected and preserved for the period April 1997−March1998 and then November 1998 to the present.

In addition to the historic biogeochemical sampling,monthly sampling along the Port Hacking transect (Table I)includes hydrographic measurements, Conductivity, Temper-ature and Depth (CTD) profiles, water quality, chlorophyll

0-933957-39-8 ©2011 MTS

Site ID Latitude Longitude Depth DistS E (m) (km)

PH025 S34◦04.94′ E151◦10.79′ 25 0.9PH050∗ S34◦05.35′ E151◦11.35′ 50 2.1PH100∗@ S34◦06.98′ E151◦13.14′ 100 6.1PH125 S34◦08.88′ E151◦15.37′ 125 11.1

TABLE IDETAILS OF THE BIOGEOCHEMICAL SAMPLING SITES.∗ CSIRO FUNDED

SAMPLING FOR TEMPERATURE AND NUTRIENTS. @ IMOS FUNDEDSAMPLING FOR CHLOROPHYLL-A, PIGMENTS (HPLC) CDOM, PAR,

PHYTOPLANKTON AND ZOOPLANKTON.

sampling at PH050 and PH100 plus a plankton net sample anda water sample for genetic analysis at PH100. It is intendedthat monthly sampling along the NSW-IMOS transect offPort Hacking also serves to calibrate fluorometric observationsobtained from the instrumented moorings and ocean colour es-timates of chlorophyll obtained from satellites (e.g. MODIS):chlorophyll, total suspended solids and coloured dissolvedorganic material (CDOM).

151 05’E 151 10’E 151 15’E 151 20’E 151 25’E 151 30’E 34 10’S

34 05’S

34 00’S

33 55’S

33 50’S

33 45’S

ORS065

SYD100

SYD140

PH100

150 E 151 E 152 E 153 E 154 E 155 E

37 S

36 S

35 S

34 S

33 S

32 S

31 S

30 S

29 S

100

100

200

20

00

Sydney

Co!s Harbour

Eden

Jervis Bay

Narooma

Tasman Sea

T, ADCP

T, ADCP, WQM

T

Sampling

Wave Rider

Proposed

Fig. 1. Map of NSW-IMOS region showing location of NSW on the Eastcoast of Australia. Inset shows the Sydney study site. Symbols denote thevarious moorings T - Temperature, ADCP - velocity, WQM - temperature,salinity, pressure, fluorescence, turbidity and dissolved oxygen. The hydro-graphic sampling sites are also marked (+).

2) Moored Observations: To build on our long history ofobservations at the 50 m and 100 m Port Hacking stations, theNSW-Integrated Marine Observing System (NSW-IMOS) de-ployed an array of moorings shore normal off Bondi. Presentlythe SYD100 and SYD140 moorings (Table II) consist of abottom mounted TRDI 300 kHz ADCP housed in a rigid framewith gimbal mount and a string of Aquatech 520 temperatureand temperature/pressure loggers at 8 m intervals through thewater column. The line of thermistors is supported by a float,approximately 20 m below the surface. Below the sub-surfacefloat at SYD100 is a Wetlabs water quality meter (WQM)that consists of a SeaBird CTD, as well as measurements of

Platform Latitude Longitude Depth Dist DeployedCode S E (m) (km)CH070 30◦17.00′ 153◦17.91′ 70 15 Aug 2009CH100 30◦16.07′ 153◦23.80′ 100 22.2 Aug 2009ORS065 33◦53.88′ 151◦18.9′ 65 2.1 1989SYD100 33◦56.63′ 151◦23.03′ 100 9.9 Jun 2008SYD140 34◦00.08′ 151◦27.92′ 140 19 Jun 2008PH100 34◦06.98′ 151◦13.14′ 100 6.1 Nov 2009BMP090 36◦11.5′ 150◦14.0′ 90 8.9 Mar 2011BMP120 36◦12.9′ 150◦18.50′ 120 16.1 Mar 2011

TABLE IIDETAILS OF NSW-IMOS MOORING SITES

fluorescence, and turbidity (Wetlabs FLNTU) and dissolvedoxygen. These moorings were initially deployed in June 2008and apart from some data losses due to instrument failure acomprehensive data set has been returned. In November 2009an additional mooring was deployed at PH100 (Table II). In-strumentation was added in a staggered fashion as the securityof the site became trusted. The mooring is now configuredidentically to the SYD100 mooring, with the inclusion of astand alone surface float, allowing for the measurement oftemperature immediately below the surface. Temperature andvelocity data are recorded at 5 min intervals while the WQMrecords 60 burst samples at a rate of 1 Hz, every 15 mins. ThePort Hacking site is one of a series of nine national referencestations located around Australia [13].

III. RESULTS

We undertake comparisons of the monthly CTD profilingdata and the WQM timeseries to assess how representativethe profiling data is of the variability in the coastal ocean offPort Hacking. Figures 2 and 3 show the monthly averagesin temperature and salinity respectively, measured at PH100(vertical CTD cast data averaged around 5 m above andbelow the depth of the WQM). Also shown is the monthlyaveraged data calculated from the high resolution WQM dataat SYD100 and PH100. The monthly CTD profiles do asurprisingly good job at representing the seasonal variability inboth the temperature and salinity fields. While the sub-monthlyvariability is highest in the summer temperature fields, themean monthly temperature is within less than 2◦C or 8% ofthe temperature. During winter time these differences decreaseto 2%. Alongshore variability is evident with the SYD100 data(nearly 20 km to the north of the PH site) exhibiting moresummertime variability and hence a greater anomaly whencompared to the PH vertical profiles. During winter howeverthe CTD profiles at the PH100 site are within 0.5◦C of themean temperature data obtained from averaging the 15 minutedata. This gives significant credibility to the monthly CTDprofiles as being representative of the coastal environment overthe seasonal timescales.

Interestingly variability in salinity is even less than that oftemperature. Previously it was thought that the PH site may

have some estuarine influence as it is located offshore fromthe mouth of the Port Hacking estuary. If there is an estuarineinfluence, it is not evident in the salinity signal. The salinityanomaly between the SYD and PH sites is at most 0.1 andgenerally less than 0.1% (Figure 3). This is an incrediblypositive result. Clearly monthly sampling is appropriate torepresent the seasonal and annual salinity conditions at a depthof 20 m below the surface in this dynamic continental shelfregion. The data also shows that higher frequency variabilityis negligible in the salinity signal.

Fig. 2. Monthly averages in temperature measured at PH100 (Vertical CTDcast averaged around the depth of the WQM). Also shown is monthly averageddata calculated from the high resolution WQM data at SYD100 and PH100.Panel 2 shows the difference between the WQM and CTD observations. Panel3 expresses the difference as a percentage.

Fig. 3. Monthly averages in salinity measured at PH100 (Vertical CTD castaveraged around the depth of the WQM). Also shown is monthly averageddata calculated from the high resolution WQM data at SYD100 and PH100.Panel 2 shows the difference between the WQM and CTD observations. Panel3 expresses the difference as a percentage.

A. Interpreting bio-optical signals

While undertaking the analysis on the fluorescence andturbidity data we identified issues with the comparison offluorescence records from one sensor to another. We identifiedstep changes in the records of both fluorescence and turbidityas measured by the WQM. Wetlabs expressed that variabilitybetween sensors could be as much as 40%. (I. Walsh, Pers.Comm.). This points to the need for very careful calibrationpre and post deployment and research is ongoing over thebest calibration technique (M. Doblin Pers. Comm.). Whilesome techniques are suitable for tying an instrument to itself(i.e identifying sensor drift), it is a lot more challenging tocompare instruments with each other.

Biofouling does not appear to be an issue here as thedeployments have been short enough that significant algalgrowth does not occur. The WQMs are also equipped withvarious anti-biofouling systems. However this is certainly anissue for ongoing consideration.

As the florescence and turbidity data is still undergoingquality control, it is not yet possible to assess the value of theFLNTU data obtained from the WQM sensors in the contextof a long term observing system. However is is clear that whileannual calibrations may be sufficient for traditional physicalmeasurements of temperature and salinity, it is clear that verycareful pre and post deployment calibrations are essential toensure the integrity of the bio-optical data.

IV. DISCUSSION

The long timeseries of CTD profile data (10 yrs since 1997)clearly shows the seasonality of the EAC identified by warm,saline water (Figure 4) and various cold water, lower salinity(35.35) intrusion events. Associated with the summer peaksin warm water is a sub surface deep chlorophyll maximum(DCM). The average depth of the DCM is approximately30 m, however at times this high chlorophyll water extendsto the surface and occasionally extends to a depth of ap-proximately 60 m. Given the discussion above regarding thereliability of various fluorescence sensors one is tempted toquestion the reliability of the absolute chlorophyll readings.The patchiness associated with the fluorescence peaks suggeststhat a point measurement of fluorescence by a single WQMis not appropriate for capturing the vertical variability inthe chlorophyll sensor. With the advancements in mooringtechnology it is suggested that if we can overcome the issueswith the reliability of fluorescence sensors, then profiling in-struments are needed to capture the complex vertical structure.

In summary, the sub-monthly variation in temperature, andsalinity are very insignificant compared to the monthly andseasonal signal. Monthly physical sampling appears adequateto capture the large scale variability in temperature and salin-ity. Initial conclusions from the fluorescence timeseries indi-cate that there is significant sub-monthly variability howeverit is noted that vertical variability needs to be captured, whichpoints to a need for profiling instrumentation. These data havealso highlighted the need for careful calibration of opticalsensors.

Fig. 4. Historic CTD cast profiles showing temperature, salinity andfluorescence (factory calibration for chlorophyll). Black triangles indicatetiming of the CTD casts.

ACKNOWLEDGMENT

IMOS is supported by the Australian Government throughthe National Collaborative Research Infrastructure Strategyand the Super Science Initiative. We are grateful to TimIngleton and the team at NSW OEH who collected andprovided the historic CTD cast data.

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