Characterizing estuarine plume discharge into the coastal ocean using fatty acid biomarkers and pigment analysis.

Marine Environmental Research xxx (2014) 1e11

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Marine Environmental Research journal homepage: www.elsevier.com/locate/marenvrev

Characterizing estuarine plume discharge into the coastal ocean using fatty acid biomarkers and pigment analysis Andrew M. Fischer a, b, *, John P. Ryan a, Christian Levesque a, c, Nicholas Welschmeyer d a

Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039, USA National Centre for Marine Conservation and Resource Sustainability, University of Tasmania, Australian Maritime College, Launceston, TAS 7250, Australia c John Abbot College, 21 275 Lakeshore Road, Sainte-Anne-de-Bellevue, Québec H9X 3L9, Canada d Moss Landing Marine Laboratories, 8272 Moss Landing Road, Moss Landing, CA 95039, USA b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 August 2013 Received in revised form 4 April 2014 Accepted 13 April 2014

The transformation of estuaries by human activities continues to alter the biogeochemical balance of the coastal ocean. The disruption of this balance can negatively impact the provision of goods and services, including fisheries, commerce and transportation, recreation and esthetic enjoyment. Here we examine a link, between the Elkhorn Slough and the coastal ocean in Monterey Bay, California (USA) using a novel application of fatty acid and pigment analysis. Fatty acid analysis of filtered water samples showed biologically distinct water types between the Elkhorn Slough plume and the receiving waters of the coastal ocean. A remarkable feature of the biological content of the plume entering the coastal ocean was the abundance of bacteria-specific fatty acids, which correlated well with concentrations of colored dissolved organic matter (CDOM). Pigment analysis showed that plume waters contained higher concentrations of diatoms and cryptophytes, while the coastal ocean waters showed higher relative concentrations of dinoflagellates. Bacteria and cryptophytes can provide a source of labile, energy-rich organic matter that may be locally important as a source of food for pelagic and benthic communities. Surface and depth surveys of the plume show that the biogeochemical constituents of the slough waters are injected into the coastal waters and become entrained in the northward flowing, nearshore current of Monterey Bay. Transport of these materials to the northern portion of the bay can fuel a bloom incubator, which exists in this region. This study shows that fatty acid markers can reveal the biogeochemical interactions between estuaries and the coastal ocean and highlights how man-made changes have the potential to influence coastal ecological change. Ó 2014 Elsevier Ltd. All rights reserved.

Keywords: Fatty acid markers Pigment analysis Estuaries Coastal zone Bacteria Phytoplankton Ecosystem change

1. Introduction Estuaries are regions of transition from terrestrial to ocean environments and areas of intense human activities. Human transformation of these transition zones has and continues to influence the input of land-based contributions to the coastal ocean (Howarth, 2008; Lotze et al., 2006; Daoji and Daler, 2004; Jickells, 1998). Coastal waters provide numerous goods and services including fisheries, commerce and transportation, recreation and esthetic enjoyment. Alteration of these environments can

* Corresponding author. National Centre for Marine Conservation and Resource Sustainability, University of Tasmania, Australian Maritime College, Launceston, TAS 7250, Australia. Tel.: þ61 3 63243802; fax: þ61 3 63243804. E-mail addresses: [email protected], afi[email protected], andy.fischer@utas. edu.au (A.M. Fischer), [email protected] (J.P. Ryan), [email protected] (C. Levesque), [email protected] (N. Welschmeyer).

negatively impact these uses. Alterations can impact coastal ocean salinity and turbidity patterns, nutrients and dissolved oxygen concentrations. These, in turn, influence productivity, structure, and behavior of coastal phytoplankton and animal populations. Increased pollutant and nutrient input to coastal ocean can also stimulate phytoplankton growth and production, sometimes resulting in nuisance or harmful algal blooms (Yentsch et al., 2008; Hu et al., 2004; Anderson et al., 2008). The characterization and quantification of transport pathways and biological materials across the land-sea interface is important for understanding impacts of estuarine alterations to the ecology of coastal and nearshore marine environments. The Elkhorn Slough is a small tidally-forced estuary in Central California (Fig. 1). It extends inland 11.4 km from Monterey Bay and is surrounded by a watershed area of 182 km2. Rapid man-made changes have significantly altered the hydrological structure and functioning of the slough. In 1946, the Army Corp of Engineers

http://dx.doi.org/10.1016/j.marenvres.2014.04.006 0141-1136/Ó 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Fischer, A.M., et al., Characterizing estuarine plume discharge into the coastal ocean using fatty acid biomarkers and pigment analysis, Marine Environmental Research (2014), http://dx.doi.org/10.1016/j.marenvres.2014.04.006

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A.M. Fischer et al. / Marine Environmental Research xxx (2014) 1e11

Fig. 1. The location of the Monterey Bay and the Elkhorn Slough (left panel). The right panel depicts the Elkhorn Slough (ES) outflow into Monterey Bay. Green indicates coastal wetland. The underway mapping grid is shown as a black line. The autonomous underwater vehicle sample area is shown as a red triangle. Water sampling locations are shown in the gray ovals, C ¼ coastal water, I ¼ intermediate water, and P ¼ plume waters. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

constructed Moss Landing Harbor at the head of the Monterey Submarine Canyon by cutting through the dune barrier that separated the slough from Monterey Bay. As a result, the slough was transformed from a sluggish estuarine lagoon that experienced periodic exchange with the coastal ocean, to a tidally-forced marine inlet that is now directly linked to Monterey Bay (Broenkow and Breaker, 2005). The hydrography of the estuary has changed from a fresh-water brackish environment, which was primarily depositional, to a saltwater environment that is erosional (Broenkow and Breaker, 2005). This has widened the main channel and increased the tidal prism, the amount of water exchanged between the slough and Monterey Bay, in one tidal cycle. In addition, tides in the slough are asymmetric, producing ebb currents that are stronger than flooding currents (Broenkow and Breaker, 2005). As a result, while the flood tide introduces relatively clear water from Monterey Bay, the waters discharged during the ebb tide are laden with sediment eroded from the banks and bottom of the slough. In addition, runoff during rainy periods can introduce agricultural fertilizers, pesticides and other materials into the plume waters from the surrounding watershed. Fatty acids (FA) can be used to trace the source and fate of biological materials in aquatic ecosystems (Mortillaro et al., 2012; Wessels et al., 2012; Burns et al., 2011; Allan et al., 2010; Parrish et al., 2005; Copeman and Parrish, 2003; Cranwell, 1982). In particular, they have been utilized to denote the relative contribution of diatoms, bacteria, dinoflagellates or terrestrial organic matter in marine environments (Meziane and Tsuchiya, 2002; Zou et al., 2004). Individual fatty acid compounds, groups of compounds and FA ratios can be used to indicate the presence and/or sources of certain types of organisms or biological material. Generally, long chain (>C20) saturated fatty acids have been associated with vascular plants and are thought to originate from terrestrial sources (Cranwell, 1982). The external surface of many

higher terrestrial plants are comprised of a cuticular layer covered by a waxy deposit rich in long chain (>C20) fatty acids. The fatty acid 26:0 is commonly used as an indicator of allochthonous terrestrial plant inputs in marine systems (Wang et al., 2013; Whiles et al., 2010) while linolenic acid (18:3), a kind of omega-3 fatty acid found in plants, has also been associated with terrestrial plant inputs (Copeman and Parrish, 2003). The sums of isoand anteiso-branched chain fatty acids, 15:0 and 17:0 have been used to trace the presence of bacteria in aquatic systems (Fulco, 1983; Sargent et al., 1987). Bacteria specifically synthesize a series of odd-number C15eC17 (branched and normal) and also 18:1n-7 fatty acids, which are associated with membrane phospholipid material (Parrish et al., 2005; Fulco, 1983; Sargent et al., 1987; Kaneda, 1991). Also, specific polyunsaturated fatty acids (PUFAs) suggest the presence phytoplankton material in aquatic samples. Various phytoplankton produce a variety of PUFAs, including 20:4, 20:5, and 22:6 (Claustre et al., 1988; Volkman and Cambie, 1989). Phytoplankton also produces saturated fatty acids (SFA) and monounsaturated fatty acids (MUFA) such as 16:0 (Sargent et al., 1981) and 16:1n-7 (Nichols et al., 1993). A relatively high ratio of 16:1n-7 to 16:0 is often used as an algal input index (Volkman and Cambie, 1989), and diatoms have been indicated by increased proportions of 20:5n-3 or by an elevated ratios of S16:1/16:0 (Claustre et al., 1988; Budge and Parrish, 1998). The ratio of 16:1n-7/ 18:4n-3 has been used as an index of diatom versus flagellate biomass, a high ratio indicating a predominance of diatoms (Sargent et al., 1987; Stubing and Hagen, 2003). Pigment-related methods are also a useful tool in determining the contribution of individual taxonomic groups to populations of phytoplankton. Chlorophylls and carotenoids are often used as chemotaxonomic biomarkers (Gutierrez-Rodriguez and Latasha, 2011) and can serve as tracers of important biogeochemical processes including taxon-specific algal growth rates (Welschmeyer



et al., 1991), taxon-specific grazer selectivity (Strom and Welschmeyer, 1991) and in the present context, as taxon-specific tags for distinct sources of natural water (Aneeshkumar and Sujatha, 2012; Williams and Claustre, 1991). The most common technique for the analysis of chlorophylls and carotenoids in aquatic ecosystems is High Performance Liquid Chromatography (HPLC) (Bianchi et al., 1995; Jeffery et al., 1999). Tester et al. (1995) used HPLC-derived pigment profiles to assess the composition and dynamics of phytoplankton assemblages in the Newport River Estuary (North Carolina, USA), concluding that HPLC-based pigment analysis is a useful tool in the highly dynamic estuarine environment. Similarly, Paerl et al. (2010) examined ecological change in the Neuse River estuary (North Carolina, USA) using HPLC-based pigment analysis, finding that freshwater input strongly interacted with nutrient input and temperature to determine the composition of phytoplankton biomass. The common practice for algal community analysis is to utilize diagnostic biomarker pigments, characteristic of specific algal groups, and calculate the relative proportion that each group contributes to the total community matrix. The popular CHEMTAX program (Mackey et al., 1996) provides a convenient iterative procedure to derive the relative contribution that each pigment-based group contributes to the total chlorophyll analyzed in any given sample. The results of CHEMTAX analysis were used in the present study to identify source water from Elkhorn Slough to complement the corresponding analysis of fatty acid biomarkers made on the same samples. The primary aim of this study was to assess the practicability of using fatty acid biomarkers and algal pigments to characterize the organic matter exchange between Elkhorn Slough and Monterey Bay. Here we assess the feasibility of using fatty acid and chemotaxonomic biomarkers to characterize the concentrations of terrestrial, phytoplanktonic and bacterial constituents in a tidal plume leaving an estuarine lagoon, the Elkhorn Slough, as well as in the receiving coastal waters of Monterey Bay, California. Combined with surface and sub-surface surveys from underway mapping systems and an autonomous underwater vehicle, the ecological consequences of this exchange were further examined. 2. Materials and methods 2.1. Sample collection and filtration Surface water samples (n ¼ 12) were collected using opaque and clean high density polyethylene sampling bottles from a Boston Whaler during the ebb tide on January 6, 10, 20 and 21, 2005 at three locations: in the plume as it immediately exits the harbor entrance (plume water), at locations offshore (coastal waters), and between the plume and coastal waters (intermediate waters), where plume waters exiting the slough have mixed with coastal waters (Fig. 1). The plume and coastal waters were examined during the winter months when rainfall, and consequently freshwater runoff from land, is the highest. The mean change in tidal height, from highehigh to lowelow tide, during the sampling period was 1.9 m (6.23 feet) while during the month of January the mean change was 1.5 m (5 feet). Sampling bottles were filled from the front of the boat carefully avoiding engine exhaust, fuel and lubricants. Nitrile gloves were used to minimize sample contamination. Water samples for both fatty acid and pigment analyses were stored on ice in a dark environment (closed cooler) until they were returned to the laboratory ( 0.05, Table 3). SFA concentrations between plume and intermediate waters were not significantly different while those between plume/coastal and intermediate/coastal waters showed a significant difference (Table 4). MUFA concentrations were significantly different between plume and coastal waters but not between intermediate and plume or coastal waters. The highest level of significance between water types occurred in MUFA’s and bacterial and phytoplankton markers. An example of a chromatograph for a coastal water sample (Fig. 2) show a typical chromatogram of coastal waters plotted against the 37 Component FAME mix standard (Supleco, USA). Bacterial FA marker concentrations in plume waters were significantly greater than those in coastal and intermediate waters, while concentrations between intermediate and coastal waters were not significantly different (Fig. 3; Table 3). Terrestrial matter showed no significant differences between plume, coastal or intermediate waters. Differences in the algal input biomarker were not significantly different between any of the water types. Diatom FA biomarker (S16:1/16:0) ratios showed a significant difference between plume and coastal waters, as well as intermediate waters, but there was no significant difference between intermediate and


A.M. Fischer et al. / Marine Environmental Research xxx (2014) 1e11 Table 1 Total lipid content (mg L1) and fatty acid composition (mg L1) for Elkhorn Slough plume, receiving coastal and intermediate waters. Values represent the mean SE of four different samples. Fatty acids

Coastal (n ¼ 4)

Total fatty acid content (mg L1) 12:0 14:0 15:0 iso16 16:0 16:1n-9 16:1n-7 16:1n-5 iso17 16:2n-4 17:0 17:1 18:0 18:1n-9 18:1n-7 18:2n-6 18:3n-4 18:3n-1 18:4n-3 18:4n-1 20:0 20:1n-11 21:0 20:4n-3 20:5n-3 22:0 21:n-9 23:0 22:4n-6 22:5n-6 24:0 22:5n-3 22:6n-3 25:0 26:0 Unidentified

57.4 3.40 1.95 0.27 1.16 9.90 0.34 2.15 0.32 0.55 0.44 0.57 0.60 2.81 1.90 2.03 0.90 0.88 0.95 1.38 0.15 0.60 0.92 0.78 0.83 3.11 0.09 0.48 0.50 0.16 0.11 0.34 0.19 4.45 0.37 1.25 1.33

Intermediate (n ¼ 4) 15.8 1.19 0.48 0.09 0.56 2.64 0.11 0.60 0.08 0.33 0.14 0.19 0.20 0.92 0.56 0.61 0.25 0.31 0.63 0.40 0.05 0.19 0.32 0.25 0.32 0.9 0.03 0.22 0.18 0.09 0.05 0.16 0.06 1.25 0.2 0.46 0.29

104.7 13.9 3.44 0.53 6.07 11.4 0.68 2.71 0.82 0.29 0.54 1.47 1.41 5.98 3.26 4.41 0.93 1.92 2.22 0.89 0.60 1.38 1.79 2.50 3.45 1.92 0.32 1.08 0.82 0.47 0.37 0.38 0.37 2.74 0.48 2.93 2.52

19.2 1.98 0.43 0.26 0.98 2.83 0.14 0.85 0.09 0.41 0.03 0.31 0.34 1.7 0.52 0.71 0.24 0.34 1.1 0.25 0.74 0.18 0.31 0.33 0.46 0.83 0.11 1.25 v0.25 0.11 0.12 0.19 0.11 1.18 0.06 0.48 0.60

Plume (n ¼ 4) 129.6 11.2 3.09 1.18 4.15 20.2 0.74 5.55 0.71 0.88 0.21 1.75 1.53 12.4 4.44 7.54 1.29 1.97 2.81 1.11 0.98 1.41 1.83 2.44 2.29 3.72 0.41 2.32 0.80 0.34 0.47 0.59 0.57 4.00 0.59 2.23 2.71

19.0 6.55 0.89 0.11 2.54 2.53 0.07 0.42 0.41 0.2 0.31 0.32 0.46 2.0 0.67 0.86 0.20 0.74 0.56 0.32 0.3 0.5 0.7 1.0 1.73 0.68 0.12 0.67 0.22 0.16 0.09 0.12 0.14 0.53 0.08 1.3 0.51

coastal waters. Dinoflagellate FA biomarker (18:4n-3/16:1n-7) ratios in coastal waters showed a significant difference between plume and intermediate waters, yet there was no significant difference between plume and intermediate waters alone. Ordination by principal component analysis by site further verifies differences in total FA concentrations between water types (Fig. 4). Here, principal component analysis explained 91% of the total variance in the data using only two principal components. The first principal component (PC1) explained 51% of the total variance, while the second principal component (PC2) explained 32% of the

Table 2 Groups of compounds and ratios of fatty acids used to distinguish the presence of various forms of biological material in the plume, intermediate and coastal waters. Fatty Acids/ratios

Marker

Reference

18:2n-6 and 26:0

Terrestrial

15:0, iso17, 17:0, and 18:1n-7

Bacterial

S16:1/16:0

Diatoms

18:4n-3/16:1n-7

Dinoflagellates

Copeman and Parrish, 2003; Wang et al., 2013; Whiles et al., 2010 Parrish et al., 2005; Fulco, 1983; Sargent et al., 1987; Kaneda, 1991 Claustre et al., 1988; Budge and Parrish, 1998 Sargent et al., 1987; Stubing and Hagen, 2003

5

total variance. Coastal waters showed a negative loading along PC1 and a positive loading along PC2. Plume waters generally showed a positive loading along PC1 and a negative loading along PC2. FA concentrations of intermediate waters clustered between plume and coastal waters. 3.2. Pigment and water quality parameters Pigment analysis further complements fatty acid marker results when describing the composition of plume and coastal waters. As in the fatty acid results, pigments show that plume waters are higher in relative proportions of diatoms, while coastal waters are higher in relative proportions of dinoflagellates (Fig. 5). Furthermore, plume waters contain higher relative proportions of cryptophytes. Cyanophytes and chlorophytes, which made up a smaller component of the total pigment content of the samples, also showed a higher relative concentration in plume waters. There were also correlations between S16:1/16:0 and fucoxanthin (diatoms) and between 18:4n-3/16:1n-7 and peridinin (dinoflagellates). The ratio S16:1/16:0 correlated well with increasing concentrations of the pigment fucoxanthin, (r ¼ 0.75 p ¼ 0.05) indicating a greater contribution of diatoms in the samples. The ratio 18:4n-3/16:1n-7 correlated with pigment concentrations of peridinin, a pigment prevalent in dinoflagellates (r ¼ 0.80, p ¼ 0.03). 3.3. Underway mapping and autonomous underwater vehicle surveys Underway mapping of plume waters showed that the relative fluorescence of colored dissolved organic matter (CDOM) and the concentration of nitrate clearly distinguish the surface expression of the plume from ocean waters. Underway sampling values of nitrate and CDOM, in the vicinity of each water sampling station, were higher in plume waters and decreased by 60% and 67%, respectively, in the coastal water stations (Table 5). Likewise, low optical transmission indicates that the plume was rich in suspended sediments; the coastal waters were relatively clear in comparison. Physical properties of the plume waters (temperature and salinity) did not differ greatly, though plume expression was slightly cooler and less saline. Fatty acid biomarkers which showed significant differences between plume and coastal waters were best correlated with the plume characteristics of CDOM, percent transmission and nitrate. Bacteria-specific FA markers, as well as the diatom biomarker index, showed a positive correlation with increases in CDOM and nitrate and a negative correlation with optical transmission (Fig. 6). The dinoflagellate biomarker index showed the opposite trend, with the index increasing with decreases in CDOM and nitrate concentrations and increases in optical transmission. Mapping of surface nitrate and CDOM showed that dissolved constituents appear to be entrained in northward coastal flow after exiting the slough (Fig. 7). Within the plume, vertical sampling with the AUV shows partitioning of particulate and dissolved constituents (Fig. 8). The ratio of optical backscatter to chlorophyll fluorescence is used to describe suspended particulate matter dominated by sediments rather than phytoplankton, while nitrate is used to describe dissolved constituents. While the vertical distribution of particulates on Jan-20-2005 followed the southern flank of the AUV sampling triangle, flowing to the south and dispersing throughout the water column, near-surface water with elevated nitrate concentrations is observed on the northern flank of the sampling triangle, flowing to the north (Fig. 8).


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Fig. 2. Major classes of fatty acids, saturated fatty acids (SFA), polyunsaturated fatty acids (PUFA) and monounsaturated fatty acids (MFA) in mg L1 for the Elkhorn Slough plume, coastal and intermediate waters. Values represent the mean SE of four samples. The inset show a typical chromatogram of coastal waters (red line) plotted against the 37 Component FAME mix standard (Supelco, USA). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

4. Discussion Table 3 Statistical summary from ANOVAs comparing the relative concentrations of fatty acids among the different water types (coastal ocean, plume and intermediate); ns, not significant, *, P 0.05, **P < 0.01.

Total fatty acid composition SSFAs SMUFAs SPUFAs SBacterial markers STerrestrial markers Algal input Diatoms Dinoflagellates

F3,9

P

4.15 5.27 8.37 1.63 9.42 1.17 0.92 7.17 9.71

0.05ns 0.03* 0.008** 0.25ns 0.006** 0.359ns 0.433ns 0.014* 0.005**

Table 4 Summary of protected least squared difference. The line indicates that there was no significant difference between water types.

Coastal

Intermediate

SFAs

|

MUFAs

|____________________________|

Bacteria

|____________________________|

Diatoms

|____________________________|

Dinoflagellates

Plume

___________________________

|

|___________________________|

|___________________________|

The Elkhorn Slough (ES) exchanges waters with Monterey Bay twice daily, year round. This exchange is nearly three times greater than the discharge of all rivers into Monterey Bay. Hydrological alterations to the slough have transformed the quantity and quality of water that empties in Monterey Bay on each ebb tide (Broenkow and Breaker, 2005). The ES now represents a significant land-sea link between the surrounding watersheds and the coastal ocean with the potential to alter coastal biogeochemical cycling and nearshore ecology. This study is the first to examine the use of fatty acid and pigment analyses to characterize the constituents of the ES plume. The outgoing plume expresses itself, through fatty acid and pigment signature, as biologically distinct from surrounding coastal waters. A remarkable feature of the biological content of the plume waters is the abundance of bacteria-specific FAs (Fig. 3). It is also noticeable that the concentration of bacterial FAs correlates with concentrations of CDOM. Zou et al. (2004) showed that bacterial fatty acids are an important component of dissolved organic matter (DOM) in estuarine and coastal waters. Bacteria release their cellular components into the DOM pool through several different ways: direct release from bacterial capsular material (Stoderegger and Herndl, 1998), viral lysis of free-living and particle-associated bacteria (Furhman, 1999), and heterotrophic grazing of flagellates on bacteria (Nagata and Kirchman, 1992). The FA content of the plume samples were also predominantly SFAs. This is also consistent with the results found by Zou et al. (2004) who found that even-carbon-number, SFAs dominated high-molecular-weight dissolved organic matter samples from estuarine and coastal waters. However, contrary to Zou et al. (2004), long chain (C22eC26)



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Fig. 3. Fatty acid composition of samples collected from locations within the slough plume, coastal, and intermediate waters. Bacterial material is indicated by the sum of fatty acids 15:0, and 17:0 (iso and anteiso) and 18:1n-7. Terrestrial material is indicated by specific fatty acids 18:2n-6 and 26:0. The fatty acids ratios 16:1n-7/16:0 S16:1/16:0 and 18:4n-3/ 16:1n-7 are indicative of algal input, diatoms and dinoflagellates, respectively.

saturated fatty acids were found in Elkhorn Slough plume and coastal water samples, but made up only an average of 2.3% of the total samples. Shorter chain saturated fatty acids, especially 16:0, the most abundant fatty acid, may be present in high quantities because of its origination in numerous marine and terrestrial cellular materials. The plume exiting the slough also showed elevated nitrate concentrations in comparison to the surrounding coastal waters (Figs. 6 and 7). N-loading in the slough has been shown to be both marine and land-based (Chapin et al., 2004; Fry et al., 2003). The main land-sources of N to the slough are thought to be in the form of nitrate from agricultural fields, dairy farms, and urban surfacewater runoff. High nutrient, freshwater runoff enters the Elkhorn

Plume

Intermediate

Slough after wet season storm events with rainfall greater than 1.5 cm. The duration and magnitude of these nutrient pulses depends on whether they occur early in the rainy season when they are rapidly flushed out of the system, or later in the rainy season

Plume Chlorophytes (20%) Cryptophytes(28%)

Cyanophytes (5%)

Coastal

Dinoflagellates (19%) Diatoms (29%)

PC2 - 32%

Coastal Cyanophytes (1%)

Chlorophytes (5%) Cryptophytes (19%)

Diatoms (16%)

Dinoflagellates (60%)

PC1 - 51% Fig. 4. Principal components analysis of fatty acid data from plume, coastal and intermediate waters collected from Elkhorn Slough and Monterey Bay.

Fig. 5. The average percentage of dominant algal groups in plume versus ocean waters as determined by pigment concentration.


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Table 5 Mean values for underway surface water properties in the vicinity of the sampling locations.

Coastal Intermediate Plume

Temperature ( C)

Salinity (psu)

CDOM (rfu)

Transmission (%)

Nitrate (mM)

13.5 13.5 13.3

32.1 32.1 31.4

0.25 0.36 0.77

48.8 20.2 12.2

20.6 28.5 50.1

when they can persist for weeks (Caffrey et al., 2007). Marine sources of nitrate are thought to enter the slough in the form of internal waves carrying water from w100 m depth up the Monterey Submarine Canyon and into the lower section of the slough on every rising tide (Chapin et al., 2004). During the dry season this source accounts for up to 80e90% of the nitrogen loading into the slough. Our data show that nutrient loading within the slough can be exported to the coastal ocean via the plume and transported northward. Increased concentrations of nitrate correlated with increased concentrations of bacteria-specific FAs. Our plume samples showed that the mean concentration of vaccenic acid (18:1n-7), the characterizing FA of Nitrobacter (Lipski et al., 2001), in addition to the sum of bacterial-specific markers (Fig. 3 and Table 1), was almost 4 times higher in our plume samples than in the coastal water samples. Iriate et al. (2003) showed that bacterial production, herbivory, bacterivory and microbial community respiration were markedly high in the upper portions of shallow, tidally-influenced estuaries and that the degree of tidal flushing controlled the spatial variability of microbial abundance, herbivory, and respiration. Therefore, upper portions of the slough, which are less under the

CDOM (rfu)

2

R =0.69 P =0.01

R =0.63 P =0.02

R =-0.65 P =0.01

R =-0.59 P =0.03

R =0.67 P =0.01

R =0.53 P =0.05

R =-0.54 P =0.05

1.5 1 0.5

80 Transmission (%)

influence of tidal advection, are likely to be more productive regions in terms of microbial and primary productivity. During spring tidal flushing events, this microbial activity and primary productivity may be exported to the coastal ocean. Our sampling indicates that the average tidal range of 1.9 m during the wet season was significant for the plume to serve as a mechanism to enhance transport of microbial biomass to the coastal ocean and Monterey Bay. Welschmeyer et al. (2003) showed that the lower slough, near Monterey Bay, is dominated by a rich diversity of coastal phytoplankton including dinoflagellates, cyanophytes, green algae, and most importantly in terms of biomass, diatoms. They also showed that cryptophytes at the upper end of Elkhorn Slough can make up more than 75 percent of all the phytoplankton biomass. Microscopic analysis shows that the planktonic flora of upper Elkhorn Slough is relatively species poor, dominated by only one or two species of cryptophytes. The organically laden and dim light environment of estuaries, in general, and Elkhorn Slough more specifically, provides a suitable growth environment for cryptophytes and they were found in relatively greater concentrations in the plume exiting the slough. Cryptophytes have physiological adaptations that allow them to thrive in adverse and light-limited environments. These include photoacclimation to low light environments, mixotrophy, and motility across density gradients (Bergmann, 2004). Surprisingly, terrestrial fatty acids were not more abundant in plume waters (Fig. 3). This is an important, albeit surprising, observation which suggests that the material lost from the slough is not derived from higher terrestrial plants, but may rather be produced locally, for example, material derived from planktonic production in the slough. It is also recognized that there are additional

R =0.7 P =0.01

60 40 20

R =-0.51 P =0.06

Nitrate (μM)

100

50

0

5 10 15 Bacterial Fatty Acids (μg L-1)

0.2 0.4 0.6 Diatoms Index

0

0.5 Dinoflagellates Index

1

Fig. 6. General patterns of fatty acid concentrations versus surface water properties.



9

Fig. 7. Underway surface mapping of temperature, nitrate and CDOM fluorescence during the outgoing tide on January 20, 2005. The inset shows the stage of the outgoing tide during which each of the successive rows (1e3) of variables were sampled. The black arrows indicate surface plume transport throughout the ebb tide.

Fig. 8. Partitioning of dissolved (nitrate) and particulate (backscatter/fluorescence) constituents in the vertical structure of the slough plume during the ebb tide. The inset shows the stage of the outgoing tide during which each of the successive rows (1e3) of variables were sampled.


10


terrestrial fatty acids that were not considered (long chain FA, C26e C32) in this analysis, almost impossible to see with the column and temperature program that was used. As shown by UMS and AUV results (Figs. 6 and 7), the biogeochemical constituents of the slough waters are injected into the coastal waters and become entrained in the northward flowing, nearshore current of Monterey Bay. This flux may have several potential consequences for the ecology of the bay. The Elkhorn Slough plume brings fatty acid-rich particulate matter into the bay, providing a source of labile, energy-rich organic matter that may locally be important as a source of food for the pelagic and benthic communities. Changes in the relative abundance of diatoms, dinoflagellates, cryptophytes, chlorophytes and bacteria under increasing nitrogen loading may have important implications for the receiving coastal waters. In particular, dilution of Elkhorn Slough plume water into coastal water locally modifies the composition of the planktonic assemblage and thus probably modifies food web dynamics and biogeochemical transformation rates. The introduction of bacteria and cryptophytes can provide a food source for mixotrophic, red-tide producing populations of phytoplankton, such as Akashiwo sanguinea and Cochlodinium sp., which are present in Monterey Bay (Jeong et al., 2004, Jeong et al., 2005, Kudela et al. 2008a, 2008b). Increased concentrations of CDOM and decreased optical transmission rates due to plume sediment loading change physical conditions (optical clarity) which may favor cryptophyte proliferation and also change the concentrations of specific nutritional fatty acids. Also, Ryan et al. (2008) have identified the existence of a bloom incubator in the upwelling shadow of the northern bay. The contribution from this plume and its transport northward may play an important role in fueling this incubator. Long-term mean estimates of chlorophyll based on satellite derived fluorescence line height measurements superimposed with current drifters (Fig. 9) suggest a link between the

37

mid-Bay, in the vicinity of the Elkhorn Slough, and transport in a northwesterly direction to the northern Bay. Representative drifter tracks, which mark the surface transport of outflow from the vicinity of the mouth of the Elkhorn Slough and the Pajaro River, indicate the highest signal of FLH in the wake of these slough-based sources of input (Fischer, 2009). In addition, the relatively higher concentration of chlorophytes within slough waters illustrates the potential of macroalgal blooms to form within the slough and be exported to the coastal environment. These blooms can influence infaunal abundance (Everett, 1991), and when deposited in sediments, can lead to increased benthic respiration, nutrient regeneration and hypoxia (Caffrey et al., 2010). Specific links between plume constituents and the bloom incubator and the influence of macroalgal blooms on coastal biogeochemical processes require further research. In summary, this study validates the use of fatty acid biomarkers and pigment analysis to distinguish between plume and ocean waters. In particular, the observed correlations between S16:1/16:0 and fucoxanthin (diatoms) and between 18:4n-3/16:1n-7 and peridinin (dinoflagellates) show that these FA ratios are useful to estimate diatom and dinoflagellate abundance. Furthermore, these results highlight the potential of an important land-sea interaction or biogeochemical link between Monterey Bay and the watersheds surrounding Elkhorn Slough. First, there is evidence that microbial activity and nutrient loading in the slough can be exported to the coastal ocean during wet season high tidal pulses and possibly provide a food/nutrient source to change coastal ecology. Also, increased concentrations of diatoms and cryptophytes and higher concentrations of SFAs and MUFAs to the coastal ocean are likely to have implications for food web dynamics. Our study shows how fatty acid markers can reveal the biogeochemical interactions between Elkhorn Slough and Monterey Bay, and how man-made changes to this system have an impact on coastal waters. These in situ techniques in combination with coastal observatories and AUVs will allow for even more comprehensive understanding for biogeochemical processes in the coastal ocean and ecosystem change. Acknowledgments

36.9

This research was supported by the David and Lucile Packard Foundation. We thank MBARI AUV operations for acquisition of the AUV data, Erich Reinecker for collection of the underway sampling data, the Barry lab for help with processing the fatty acid samples, and Lawrence Yuonan for his assistance with the pigment analyses. Comments by anonymous reviewers significantly improved the manuscript.

36.8

References

36.7

36.6 FLH (mW cm-2 μ m-1 sr-1) 0 0.02 0.04

-122.05

-121.9

-121.8

Fig. 9. Mean oceanographic season (AugusteOctober) chlorophyll signal (2002e2007) derived from satellite-based fluorescence line height (FLH) measurements with drifter tracks deployed at the Elkhorn Slough outlet superimposed. The drifter tracks follow the highest FLH signal representing the highest phytoplankton abundance and the potential contribution of slough constituents on the northern bloom incubator region as described in Ryan et al. (2008). (From Fischer, 2009).

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