Do Sediment Composition and Physical Forcing Have Roles?

Estuarine, Coastal and Shelf Science 157 (2015) 42e50

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Estuarine, Coastal and Shelf Science

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Variations in the release of silicate and orthophosphate along a salinity gradient: Do sediment composition and physical forcing have roles?

* Bhanu Paudel a, , Paul A. Montagna b, 1, Leslie Adams b, 2 a Academy of Natural Science of Drexel University, 1900 Benjamin Franklin Pkwy, Philadelphia, PA 19103, USA b Harte Research Institute for Gulf of Mexico Studies, Texas A&M University-Corpus Christi, 6300 Ocean Drive, Unit 5869, Corpus Christi, TX 78412, USA article info abstract

Article history: It was hypothesized that sediment composition, i.e. organic matters and minerals, and physical forcing Received 8 September 2014 can influence retention and release of silicate (SiO4) and orthophosphate (o-PO4) along salinity gradients. Accepted 18 February 2015 An experiment was performed to measure nutrient release by using treatments with and without Available online 26 February 2015 sediment organic matter from the Guadalupe and Nueces Estuaries at five different salinities. The sample mixtures were shaken at intervals over the course of 48 h to simulate wind and river forcing. The release Keywords: of silicate from sediments increased with time from 2 min to 48 h in all five salinities. The added silicate orthophosphate concentration was adsorbed in most of the sediment containing organic matter and orthophosphate calcium-rich shells from both estuaries. From the sediments without organic matter, the release of quartz m calcite orthophosphate was as high as 52 mol/L. The sediment minerals quartz and calcite were abundant in organic matter both estuaries. The average quartz to corundum peak intensities ratio were 14.04 and 13.36 and the average calcite to corundum peak intensities ratio were 3.06 and 1.32 in the Guadalupe and Nueces Estuaries respectively. The average organic matter in the Guadalupe and Nueces estuaries were 10.67% and 13.39% respectively. The retention and release of orthophosphate from the sediments may have been caused by the bonding with organic matter and calcite in the sediments. These findings indicate that sediment composition was a significant contributor in the low dissolved orthophosphate concentration in the estuaries. The release of silicate from the sediments containing quartz, and organic matter, when shaken, indicate that the combined forcing of river and wind may have been maintaining the estuaries silicate concentrations. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction According to previous studies, transported dissolved silicate decreased in concentration as it moved from a river to an estuary The fluctuation of silicate and phosphate concentrations along salinity gradient (Morris et al., 1981; Conley and Malone, 1992; Eyre estuarine salinity gradients might be due to the interaction be- and Balls, 1999). Different explanations have been suggested for the tween sediment composition and overlying water (Morris et al., decrease in silicate along river-mouth to estuary-ocean salinity 1981; D'Elia et al., 1983; Anderson, 1986). The sediment composi- gradients. Those include coagulation of a colloidal form of silica tion includes sediment organic matter and minerals that interact in (Krauskopf, 1956), retention of silicate on clay minerals as it moves the presence of salt water electrolytes. This, in turn, may affect from freshwater to the salt water interface (Williams and Crerar, retention-release, precipitation-dissolution and flocculation- 1985), and silicate uptake by diatoms (Conley and Malone, 1992). deflocculation of dissolved silicate and phosphate. A sudden increase in silicate concentration in oligohaline (salinity of 0.5e5, Venice Classification system; Anonymous, 1958) regions from riverine freshwater concentrations may be a result of the

* Corresponding author. dissolution of biogenic silica present in river water (Anderson, E-mail addresses: [email protected] (B. Paudel), paul.montagna@ 1986). tamucc.edu (P.A. Montagna), [email protected] (L. Adams). Previous studies have revealed two types of phosphate behavior 1 Tel.: þ1 361 825 2040. in estuarine salinity gradients. The first one is the decrease in 2 Tel.: þ1 361 825 2033. http://dx.doi.org/10.1016/j.ecss.2015.02.011 0272-7714/© 2015 Elsevier Ltd. All rights reserved. B. Paudel et al. / Estuarine, Coastal and Shelf Science 157 (2015) 42e50 43 dissolved phosphate concentration from fresh to saline water gra- 2.2.1. Organic matter dients with most of the losses at the confluence of a river and an The organic matter content in the sediment sample was deter- estuary. There are several potential causes of this sudden decrease mined by H2O2 digestion (Schumacher, 2002). H2O2 is an oxidant; in concentration; a change in pH (Garcia-Luque al. 2006), precipi- its addition oxidizes organic matter and decreases the pH (up to tation of phosphate-colloid (Bale and Morris, 1981), and retention 3e4). The decrease in pH could dissolve calcite and calcium car- of phosphate onto suspended clay particles (Garcia-Luque al. 2006). bonate shells that can release calcite bonded phosphorus The second type of phosphate behavior was the increase in phos- (Staudinger et al., 1990). Care was taken to avoid excessive frothing phate concentration in the oligohaline regions because of the and dissolution of calcite mineral. Our main goal on using H2O2 was release of phosphorus from the iron and aluminum oxide organic to digest organic matter and observe nutrient release from complex as the river water mixes with the estuary water sediments. (Sundareshwor and Morris, 1999). Flocculation of organic matter A10e15 g sediment sample was weighed and dried overnight at inhibits release from organic complexes as sediment is transported 60 C in an aluminum pan. After drying, the samples were to mesohaline (salinity of 5e18, Venice Classification system; reweighed and each placed in a beaker containing 50 ml of 30% Anonymous, 1958) and polyhaline (salinity of 18e30, Venice Clas- H2O2. The sediments from both estuaries contained clam and oyster sification system; Anonymous, 1958) regions, and result in the shells, which might get corroded by H2O2 solution. Effort was made decrease in phosphate concentration in the oceanic end of an es- to preserve sediment characteristics by not keeping the sediment tuary (Howarth et al., 1995; Nielson et al., 2001). for too long in the solution. After the digestion of organic matter the Recent research has found a salinity of 35 can dissolve silica gel supernatant was decanted and an aliquot of deionized (DI) water and release silicate in the water (Tanaka and Takahashi, 2013). was added. The beaker was stirred and allowed to settle until the Additionally, Spagnoli and Bergamini (1997) identified the release supernatant was clear. This process was repeated two more times. of phosphate during resuspension. The interaction between sedi- The sample was then filtered by pouring the beaker contents ment composition and water in the estuaries could be important in through a 0.7 mm filter paper suspended in a glass funnel. The the retention and release of silicate and orthophosphate from the sediment collected on the filter paper was oven dried at 60 C sediment. A weekly nutrient study in Nueces and Corpus Christi Bay overnight. The samples were again weighed. Sediment organic identified very low orthophosphate (o-PO4) concentrations matter was calculated by the difference in weight before and after (mean ¼ 0.12 mmol/L), and high silicate (SiO4) concentration organic matter digestion. From here on samples digested with H2O2 (mean ¼ 127.5 mmol/L) in all samplings (Turner, 2014). Phosphate refers to without organic matter treatment. can absorb onto clay particles resulting in reduced orthophosphate concentration (Garcia-Luque al. 2006), the low orthophosphate 2.2.2. Mineral content concentration might be because of the absorption onto clay parti- Sediment mineral has a role in fluctuating inorganic nutrient cles. Furthermore, in the Guadalupe and Nueces estuaries salinity concentration by retention and release into the mineral surface. and resuspension induced by wind or river flow may be the major The sediment without organic matter was ground using a mortar factors that cause variation in sediment nutrient release. Therefore and pestle, and 2 g of sample was mixed with 0.5 g of corundum (a- it is important to identify the effects of estuarine sediment Al2O3) in 4:1 ratio (Naehr et al., 2000). A few drops of ethyl alcohol composition at various salinities and the retention and release of were also added to the sample-corundum mixture to help in the SiO4 and o-PO4 concentrations due to resuspension. Thus, the homogenization. The prepared sample was then placed in a sample present study is to characterize silicate and orthophosphate release holder of a Rigakut Ultima III X-ray diffractometer and sediment from sediment in a laboratory setting by varying sediment organic mineral content was identified. In the x-ray diffractometer, scans matter, salinity and agitation rate. Agitation was used to mimic were run from 2 to 80 2q with scanning speed of 0.01 2q/s. The river flow and wind forcing. crystal structure of each mineral is composed of sets of planes responsible for ‘d’ values (distance between array of crystal in 2. Materials and methods mineral), and which is responsible for the variations in the peak. The peak, obtained as intensity count per second of the mineral 2.1. Sediment sample collection sites and bottom water quality type was compared with the corundum peak, and the mineral content was identified based on the mineral to corundum ratio. The top 5 cm of sediment were collected using cores from sta- MDI Jade e 7 software (Bishop et al., 2011) was used to identify tion ‘A’ of Guadalupe and Nueces Estuaries during July 2012, mineral types and peak intensities. The software can automatically October 2012, and January 2013 quarterly samplings (Fig. 1). Data identify the mineral content in the sample based on the location of collected from station A of both estuaries was used because it was the peak. However, the user has to differentiate the mineral type closer to the river mouth, i.e. near to the head of the estuaries, and based on the ‘d’ value and one main peak of the mineral. was therefore assumed to have higher organic loads. In the field, sediment samples were kept on ice and stored frozen upon transfer 2.2.3. Preparation of nutrients spike solution to the lab in order to slow organic matter degradation. Organic Primary standards concentration of 10,000 mmol/L were pre- contents, mineral contents and nutrient release experiments were pared using 1.36 g of potassium phosphate monobasic (KH2PO4) performed by using these sediment samples. and undried 2.12 g of sodium metasilicate pentahydrate (Na2SiO3). Temperature, pH, salinity, dissolved oxygen (DO), were collected Each chemical was weighed out, placed in 1000 ml volumetric using YSI sonde in the bottom water (20 cm above the sediment flask, dissolved and brought up to 1000 ml with freshly drawn surface) of the two estuaries. Water samples were also collected for Milli-Q water. From these refrigerated primary standards, a 100 ml nutrient analysis. mixed stock standard was prepared daily containing 500 mmol/L, o- PO4 and 5000 mmol/L SiO4. That solution was then used to ‘spike’ 2.2. Sediment analysis and laboratory experiments the solute with additional nutrients in the following experiments.

The laboratory experiment was conducted at room temperature. 2.2.4. Experimental design for laboratory study Before the sediment was used for laboratory experiment, organic All the laboratory experiments to identify sediment nutrient matter, and mineral analyses it was carefully homogenized. release were performed within two weeks of sample collection. The 44 B. Paudel et al. / Estuarine, Coastal and Shelf Science 157 (2015) 42e50

Fig. 1. Map of the study area.

laboratory experiment was conducted using sediments with and five salinity solutions of 0 (DI water), 7.5, 15, 25, and 35 were pre- without organic matter treatments at five salinity solutions over a pared using low nutrient sea water (LNSW) and DI water. During period of 48 h. A total of 6 laboratory experiments (3 with each experiment the sediment to salinity solution ratio was 1:30 by organic þ 3 without organic) using the Nueces Estuary and 6 lab- mixing 4 g of sediment in 120 ml of solution in a 250 ml poly- oratory experiments (3 with organic þ 3 without organic) using the carbonate Erlenmeyer flask. The experiment was conducted with Guadalupe Estuary sediment samples were conducted (Fig. 2). The two replicates for each salinity solutions from both estuaries for a

Fig. 2. Experimental Design for the laboratory experiment, and only Guadalupe Estuary (GE) is showing. Design repeated for Nueces Estuary (NC). T1 ¼ 1 min, T2 ¼ 15 min, T3 ¼ 50 min, T4 ¼ 360 min, T5 ¼ 48 h, ¼ replicates. B. Paudel et al. / Estuarine, Coastal and Shelf Science 157 (2015) 42e50 45 total of 20 flasks. Each flask was supplemented with the spike concentrations, whereas salinity was significantly higher in the nutrient solution to provide an additional concentration 5 mmol/L Nueces Estuary compared to the Guadalupe Estuary. Silicate con- of o-PO4 and 50 mmol/L of SiO4. centration in both estuaries stations near (Stations A and B) to the The spiked salinity solutions were carefully mixed prior to river was greater than 100 mmol/L (Table 1). adding the weighed sediment to each flask. After sediment addi- e fl tion, 3 5 ml sample was collected from each ask, using a large 3.2. Silicate and orthophosphate concentrations in the laboratory bore plastic syringe, to be used as the baseline measurement (T1). experiment Additional water samples for nutrient analysis were then collected at 10 min (T2), 50 min (T3), 360 (T4) mins and 48 h (T5) after the In the lab, SiO4 concentration was the highest in the treatments addition of sediment to the flasks. The flasks were shaken vigor- without sediment organic matter. The difference in SiO4 concen- ously using an oscillating shaker between each sampling time. The tration between Time-T5 and Time-T1 was higher in the DI treat- samples were shaken for approximately 50 s at the following in- ments without sediment organic matter compared to salinity tervals: every 2 min between T1 to T2, every 5 min between T2 to T3, treatments without sediment organic matter (Fig. 3). The average every 15 min between T3 to T4 and every hour between T4 to T5. The highest SiO4 concentrations of 120 mmol/L from the Nueces Estuary samples were allowed to settle for 10 h overnight and the agitation and 140 mmol/L from the Guadalupe Estuary were observed in the protocol was resumed the following day. DI treatments without sediment organic matter at Time-T4. The A control was prepared using the nutrient spike solution at the concentration of SiO4 was significantly different with respect to same salinity and nutrient concentration as in the experimental time, salinity, and organic content as variables, whereas it was not fl asks but without the sediment addition. The control was designed significantly different between the two estuaries (Table 2). Addi- to identify and measure the degradation of nutrients in the solu- tionally the SiO concentrations were affected more with time and fl 4 tions over the time of experiment without sediment in uence. less with organic matter and salinity. The dissolved orthophosphate The depicted SiO4 and o-PO4 concentrations were the concen- (o-PO4) concentrations in all saline treatments with sediment trations after subtracting control's concentrations at that given organic matter were below detection levels regardless of time, time. Hence the negative and positive values indicate retained and whereas this was not the case with silicate. However, the o-PO4 in released concentration to and from the sediments. some DI treatments with sediment organic matter did show detectable levels of o-PO4. Four out of ten average o-PO4 concen- 2.2.5. Inorganic nutrients analyses trations in the DI treatments with sediment organic matter were fi Three to ve milliliters of nutrient sample was taken out from greater than 0 mmol/L, i.e. ranges from 0.6 to 2.7, of which only one fl each polycarbonate Erlenmeyer ask using a syringe and each was was from the Nueces Estuary treatment (Fig. 3). The average o-PO fi m fi 4 ltered through polycarbonate 0.45 m lter paper. Each sample concentration was the highest in the DI treatments without organic was kept frozen in a labeled vial until analysis. An O.I Segmented matter. The average concentration identified in the DI treatments Flow Autoanalyzer was used for the nutrient analysis. Manufacturer without sediment organic matter from the Guadalupe Estuary and recommended applicable range of method detection limit (MDL) the Nueces Estuary were 30.2 and 24.1 mmol/L respectively (Figs. 4 are 0.02e10 mmol/L and 0.35e35 mmol/L for orthophosphate (o- and 5). In the saline treatments without organic matter, the o-PO4 PO4) and silicate (SiO4). Higher concentrations were determined by m concentration was higher in the Guadalupe Estuary treatments sample dilution. Five and 0.10 mol/L were the lower detection compared to the Nueces Estuary treatments (Figs. 4 and 5). The o- limit of silicate and orthophosphate concentrations used for pre- PO4 concentrations in the treatments without organic matter in all paring standard curve respectively. salinities increased over time (Fig. 3). The o-PO4 concentrations were significantly different with respect to organic matter, time, 2.3. Statistical method salinity, and estuary (Table 2).

Analysis of variance (ANOVA) was performed to identify the effects of salinity (SAL), time, organic matter content (OM) and 3.3. Sediment mineral and organic composition estuaries (EST) on SiO4 and o-PO4 concentration. Before the anal- In both estuaries quartz was the dominant mineral, based on the ysis, SiO4 and o-PO4 data were log transformed. ANOVA was performed using PROC GLM procedure in SAS version 9.3. mineral to corundum ratio. The average quartz to corundum ratio ® All graphs were created using SAS statistical analysis. In SAS was 14.04 and 13.86 in the Guadalupe and Nueces Estuaries different procedures of statistical and graphical analysis are per- respectively (Fig. 6). Besides quartz (SiO2) and calcite (CaCO3), e e formed using PROC syntax command. The PROC SGSCATTER and feldspar (KAlSi3O8 Na, AlSi3O8 Ca, Al2Si3O8) was also found in the PROC COMPARE procedures were performed to compare different variables. The nutrients data were checked for normality using Table 1 HISTOGRAM statement with density type ¼ kernel. Then the data Average salinity, phosphate and silicate concentrations during three ﬁeld samplings were log transformed in order to obtain normal distribution of in the Guadalupe (GE) and Nueces (NC) Estuaries. Abbreviations: DL ¼ detection ¼ ¼ residuals. PROC GCHART was performed to get bar diagrams and limit, o-PO4 orthophosphate, and SiO4 silicate.

PROC GREPLAY was used to obtain a panel of different bar diagrams. Estuary Station Latitude Longitude Salinity o-PO4 SiO4 (ppt) (umol/L) (umol/L) 3. Results mean mean mean GE A 28.36878 96.71878 11.06 2.75 183.05 3.1. Silicate and orthophosphate concentrations in the Nueces and GE B 28.37248 96.72080 15.80 1.73 160.14 Guadalupe estuaries GE C 28.31027 96.68002 22.33 1.04 117.02 GE D 28.31397 96.67832 19.75 1.22 119.25 NC A 28.39352 96.77240 34.24 2.16 149.46 In the Nueces Estuary, salinity was above 30 during all three NC B 28.34777 96.74573 35.61 1.35 113.66 samplings, whereas in the Guadalupe Estuary the lowest salinity of NC C 28.24618 96.76488 36.69 0.35 45.79 9 was identiﬁed in July 2012. The two estuaries did not vary NC D 28.30210 96.68435 36.75

Fig. 3. Average silicate and orthophosphate release from with and without organic treatments of the Guadalupe (GE) and Nueces (NC) Estuaries samples at different salinities. Y- axes of 1st and 2nd rows of the ﬁgure are silicate concentrations, and Y-axes of 3rd and 4th rows are orthophosphate concentrations. OM ¼ Organic matter (NO indicates treatments without organic matter after H2O2 digestion; YES indicates treatments with organic matter).

sediments (Table 3). Calcite was higher in the Guadalupe Estuary 4. Discussion than in the Nueces Estuary, i.e., average calcite to corundum ratio was 3.06 and 1.32 in the Guadalupe and Nueces Estuaries respec- 4.1. Dissolved silicate in the estuary and in the laboratory tively (Fig. 6). In most of the samples, magnesium carbonate (MgCO3) was less than 2% of Mg/Ca in carbonate minerals. The Guadalupe Estuary receives more inflow than the Nueces The organic matter contents in the Guadalupe and Nueces Es- Estuary (Montagna et al., 2013), which could be the reason for the tuary were almost the same in the July 2012 and January 2013 higher SiO4 concentration all stations of the Guadalupe Estuary samples, however, the organic matter content in the October 2012 compared to the Nueces Estuary. The SiO4 concentrations in the Guadalupe Estuary's sediments (3.87%) was significantly less than Guadalupe and Nueces Estuaries decreased along the river-estuary that of the Nueces Estuary's sediments (13.84%). The average to the tidal-inlet salinity gradients (Paudel, 2014). In the laboratory organic matter content in the Nueces Estuary was greater than in experiment with and without organic sediments, the SiO4 con- the Guadalupe Estuary (Fig. 6). centration decreased sharply from 0 to 7.5 salinity and then decreased steadily from 7.5 to 35 (Figs. 4 and 5). Previous studies have found a positive correlation between SiO4 and NaCl, the major components of sea water, until the concentration of NaCl reaches Table 2 1 M (equivalent to salinity of 35) (Tanaka and Takahashi, 2005, ANOVA result to determine the difference in silicate and orthophosphate concen- 2007). The decrease in solubility of SiO4 along the salinity gradi- trations by different source variables. Abbreviations: Est ¼ Estuary, OM ¼ organic ents might not be the reason for the decrease in SiO4 concentration. ¼ matter, Sal salinity. Rather, the field and lab results indicate the estuaries sediment is Main effects df Dependent variables the source and sink for silicate and orthophosphate. Silicate Orthophosphate Past studies have suggested the decrease in SiO4 concentration,

* from river water concentration, along the increasing estuary Estuaries 1 0.2084 <0.0001 * * Time 4 <0.0001 <0.0001 salinity gradient might be due to the adsorption of colloidal silicates * * Salinity 4 <0.0001 <0.0001 to the clay particles suspended in the water column (Liss and * * Organic matter 1 <0.0001 <0.0001 Spencer, 1970; Morris et al., 1981) and ﬂocculation of colloidal sil- * Denotes signiﬁcant values. icate particles in saline water (Krauskopf, 1956; Day et al., 1989). B. Paudel et al. / Estuarine, Coastal and Shelf Science 157 (2015) 42e50 47

Fig. 4. Release of average silicate and orthophosphate concentration from the Gua- Fig. 5. Release of average silicate and orthophosphate concentration from the Nueces dalupe Estuary's with and without organic treatments. Estuary's with and without organic treatments.

sediment containing silicate minerals (Rickert, 2000; Rickert et al., The dense turbidity observed in the sediment-DI water mixture 2002). The present study showed that sample agitation, in an compared to that of the sediment-7.5 and sediment-35 mixtures, attempt to mimic wind forcing and river flow, resulted in resus- after 1 h of sample shaking, indicates particulates settled faster in pension have released silicate concentrations equivalent to those the saline solutions than in the fresh solution. The particulate and colloidal silicate might have precipitated with the settled particulate matter in the saline solutions. The adsorption of SiO4 in the sediment minerals, or precipitation of particulate and colloidal silicates in the salt water, may be the possible explanation for the decrease in SiO4 concentration in the salinity gradient. During the 2010e2013 study of Nueces and Guadalupe Estu- aries, SiO4 concentration of 125 mmol/L was observed at salinity of 25 and greater, which is close to the general river water concentration i.e. 150 mmol/L identified by Conley (1997). In the laboratory experiments, the SiO4 concentration was higher in the freshwater solutions compared to that of the saline solutions. Most of the SiO4 concentrations decreased with the increase in salinity; however, in few samples the SiO4 concentrations observed at salinity 25 were higher than those observed at salinity 15. The reason for this increase was not identified. The SiO4 concentration increased from time-T1 to time-T5 in all five salinity solutions (Fig. 3). The increase Fig. 6. Sediment mineral and organic matter content in the Guadalupe (GE) and in SiO4 concentrations over time in the experiments indicate that Nueces (NC) Estuaries. Abbreviations: Iqtz ¼ peak intensity count per second of quartz, shaking the samples may release additional SiO4. Past study has Icorr ¼ peak intensity count per second of corundum (Al2O3), Ical ¼ peak intensity found that resuspension in some estuaries can release silicate from count per second of calcite. 48 B. Paudel et al. / Estuarine, Coastal and Shelf Science 157 (2015) 42e50

Table 3 Mineral contents and organic matter contents in the Guadalupe and Nueces Estuaries samples.

Sample ID Estuary Mineral content Avg. Organic contents (%) Quartz (int. cps) Calcite (int. cps) Feldspar (int. cps) Al2O3 (int. cps) Iqtz/Icorr Ical/Icorr Ifeld/Icorr d ¼ 3.342 d ¼ 3.035 d ¼ 3.181 d ¼ 3.479

GE_A_Jul12 Guadalupe 5354 980.4 173.5 339.4 15.77 2.88 0.51 15.75 NE_A_Jul12 Nueces 5596 439.5 588.7 423.8 13.2 1.03 1.38 13.46 GE_A_Oct12 Guadalupe 3571 872.7 135.4 290.9 12.27 3.00 0.46 3.87 NE_A_Oc12 Nueces 5204 513 136.4 342.1 15.21 1.49 0.39 13.84 GE_A_Jan13 Guadalupe 4058 951.7 123.3 287.8 14.10 3.30 0.42 12.41 NE_A_Jan13 Nueces 3769 467.2 2786 322.2 11.69 1.45 8.64 12.89

Int. cps ¼ peak intensity measured as count per second.

Iqtz ¼ intensity of quartz. Icorr ¼ intensity of corundum (Al2O3). Ical ¼ intensity of calcite. Ifeld ¼ intensity of feldspar. d ¼ distance between array of crystal in a mineral.

found in river water. The high dissolved SiO4 concentration might The Nueces Estuary typically has low inflow than the Guadalupe be due to the interaction between water and sediment composi- Estuary; however, the Nueces estuary has more organic matter than tion. Wind induced resuspension as observed in the laboratory the Guadalupe Estuary. That indicates recycling induced by high experiment may affect the interaction between sediment and water residence in the Nueces Estuary may have provided more overlying water resulting in the increase in SiO4 concentration. organic matter than the Guadalupe Estuary. Similar amounts of Sediment mineralogy analyses performed in the lab demon- silicate mineral content in the sediments of both estuaries may be strated that low grade silica species, quartz, and calcite are the major why the concentrations of SiO4 in the laboratory experiment were minerals found in the Guadalupe and Nueces Estuaries. The levels of not significantly different in the two estuaries. Furthermore, the quartz were similar in both estuaries, however, calcite was higher in high silicate concentration identified in the field sampling of both the Guadalupe Estuary (Fig. 6). Feldspar was detected in small estuaries and in the laboratory experiments indicates that sedi- amounts in some sediment samples, but not in all (Table 3). The ment composition was maintaining its concentration. Further study presence of low grade silica species and quartz in the Guadalupe and is still needed in order to determine the cause for high silicate Nueces Estuaries could be the reason for the high silicate concen- concentrations. tration in most of the field and lab samples. Past studies have found that at ordinary temperature the solubility of amorphous silica can 4.2. Dissolved orthophosphate in the estuary and in the lab be as high as 100 ppm while the solubility of quartz is around 6 ppm (Krauskopf, 1956). Other research has also found dissolution of In estuaries, suspended or bottom sediments' particulate quartz at 25 C, in the presence of sodium and pH of 6 (Berger et al., phosphorus is bonded with organic matter, iron, aluminum, and 1994). However, dissolution kinetics of quartz is very low and is a calcite (Upchurch et al., 1974; Lebo, 1991). The o-PO4 concentration slow process. Hence, we are not absolutely certain whether wind or in the treatments with organic matter was below zero in all the five river induced resuspension with and without salt water electrolyte samples of the 48 h lab experiment. The added o-PO4 was not may have role in the release of silicate in the water. detected in the solutions with sediments collected from the Gua- The sediment organic matter, which typically consists of dalupe and Nueces Estuaries. The loss of added o-PO4 may have plankton, animal remnants, and diatoms, had a significant effect in been due to the adsorption in the calcium carbonates shells the SiO4 concentration in the laboratory experiment (Table 2). (McGlathery et al., 1994) or in the iron particles (Froelich, 1988; Especially, treatments with organic matter have lower silicate Ingall and Jahnke, 1997), or in the organic matter (Berner and concentration than treatments without organic matter. Also inter- Berner, 1996). The phosphate adsorbing sediments in the two es- estingly, only July 2012 sample from the Nueces Estuary contains tuaries may be adsorbing enough o-PO4 to result in lower con- one diatom species i.e. Navicula Sp. Sediment organic matter may centrations identified in the long term Montagna's data set. also act as a filming effect between sediment particles and over- Sequential extraction of sediments to identify different fractions of lying water, resulting lower release in organic treatments. The phosphorus in the sediment particles would be a useful future work filming effect of sediment organic matter prevents the reaction in order to identify where in sediments (i.e. iron particles, calcite or between sediment particles and overlying water. Research has organic matter) of these estuaries phosphorus is binding with. identified dissolution of biogenic silica after removing organic Past studies have found that organic matter and iron bonded matter from the diatoms exoskeleton (Bidel and Azam, 2001; Bidel phosphorus were important in the release of phosphate to the et al., 2003). The filming effect of organic matter may be the reason water (Krom and Berner, 1981; Ingall and Jahnke, 1997; Bianchi, for the reduced level of silicate in the treatments with organic 2007). In the lab, o-PO4 was released from the treatments matter compared to those without organic matter. In the present without organic matter, which might be due to the oxidation of study effects of variable amount of organic matter on the retention/ organic matter by the addition of hydrogen peroxide. Additionally release of silicate was not observed, which is recommended in during peroxide digestion, calcite bonded phosphate might get future. Fine sized organic particles might not be affected by sample released by dissolving calcite, even though we were careful while agitation, thus, it would be hard to rule out filming effect even after organic matter digestion. For calcite bonded phosphate release a sample agitation. The removal of organic matter might have low pH condition is necessary (Staudinger et al., 1990; Bianchi, enhanced the rate of reaction between sediment particles and the 2007). Though the experimental pH was between 6.5 and 7.0, low overlying water, resulting in more SiO4 concentration in the over- pH and oxidizing environment created by H2O2 might have lying water. Most importantly, silicate could have released from released organic and calcite bonded phosphate in the treatments sediment grain size particles during constant sample agitation. without organic matter. H2O2 digestion might oxidize organic B. Paudel et al. / Estuarine, Coastal and Shelf Science 157 (2015) 42e50 49 phosphorus to dissolve phosphate thus when that peroxide treated Shanti Dhakal for helping in the laboratory work. The authors sediment kept in the solution have more o-PO4 at T1. In order to would like to thank two anonymous reviewers for their valuable have iron bound phosphate released, either a high pH i.e. close to 8, comments to improve the manuscript. or a redox condition must be satisfied (Mortimer, 1942; Gomez et al., 1999). In the present study, the release of iron bonded References phosphate was less likely because of minimal redox condition in all treatments. Additionally, another factor in the release of phos- Anderson, G.F., 1986. Silica, diatoms and a freshwater productivity maximum in phorus from sediments may be via the dissolution of calcite during Atlantic coastal plain estuaries, Chesapeake Bay. Estuar. Coastal Shelf Sci. 22, e sample agitation. In their experiment Spagnoli and Bergamini 183 197. Anonymous, 1958. The Venice system for the classification of marine waters ac- (1997) identified that phosphorus was released during resus- cording to salinity. Limnol. Oceanogr. 3, 346e347. pension by the dissolution of calcite containing sediments. The Bale, A.J., Morris, A.W., 1981. Laboratory simulation of chemical processes induced constant increase in o-PO from T to T in the without organic by estuarine mixing: the behavior of iron and phosphate in estuaries. Estuar. 4 1 5 Coast. Shelf Sci. 13, 1e10. treatment might be because of the agitation of samples containing Berger, G., Cadore, E., Schott, J., Dove, P.M., 1994. Dissolution rate of quartz in lead calcite minerals. The Guadalupe Estuary sediment samples without and sodium electrolyte solutions between 25 and 300 C: effect of the nature of organic treatments released more orthophosphate by the shaking surface complexes and reaction affinity. Geochim. Cosmochim. Acta 58 (2), 541e551. procedure than the Nueces Estuary sediments (Figs. 5 and 6). The Berner, R.A., Berner, R.A., 1996. Global Environment: Water, Air, and Geochemical presence of more calcite in the Guadalupe Estuary sediment sam- Cycle. Prentice Hall, New York. ples might be the reason for the higher orthophosphate level Bianchi, T.S., 2007. Biogeochemistry of Estuaries. Oxford University Press, NY, pp. 346e373. observed in the Guadalupe Estuary sediment compared to the Bidel, K.D., Azam, F., 2001. Bacterial control of silicon regeneration from diatom Nueces Estuary sediment. In the treatments with organic matter, detritus: significance of bacterial ectohydrolases and species identity. Limnol. despite shaking, there was no increase in the orthophosphate Oceanogr. 46, 1601e1623. fi Bidel, K.D., Brzezinski, M.A., Long, R.A., Jones, J.L., Azam, F., 2003. Diminished effi- concentration. The lming and adsorption effects by the organic ciency in the oceanic silica pump caused by bacteria-mediated silica dissolu- matter present in the sediments and bonding of phosphorus on tion. Limnol. Oceanogr. 48, 1855e1868. sediment calcium minerals may be the reason for the lack of in- Bishop, M.E., Dong, H., Kukkadapu, R.K., Liu, C., Edelmann, R.E., 2011. Bioreduction of crease in the treatment with organic matter. Fe-bearing clay minerals and their reactivity toward pertechnetate (Tc-99). Geochim. Cosmochim. Acta 75, 5229e5246. Conley, D.J., 1997. Riverine contribution of biogenic silica to the oceanic silica 5. Conclusion budget. Limnol. Oceanogr. 42, 774e777. Conley, D.J., Malone, T.C., 1992. Annual cycle of dissolved silicate in Chesapeake Bay: fi implications for the production and fate of phytoplankton biomass. Mar. Eco. The results of the laboratory experiment identi ed the impor- Prog. Ser. 81, 121e128. tance of sample agitation for the increase in silicate in the fresh and D'Elia, C.F., Nelson, D.M., Boynton, W.R., 1983. Chesapeake bay nutrient and saline solutions. Both estuaries had higher silicate concentrations at plankton dynamics: III, the annual cycle of dissolved silicon. Geochim. Cos- mochim. Acta 47, 1945e1955. higher salinities, which may be due to the effects of wind forcing, Day, J.W., Hall, C.A.S., Kemp, W.M., Yanez-Arancibia, A., 1989. Estuarine Ecology. resulting in the additional release of silicate from sediment con- John Wiley and Sons, New York, p. 558. taining silicate minerals. The long-term field study identified Eyre, B., Balls, P., 1999. A comparative study of nutrient behavior along the salinity e fl gradient of tropical and temperate estuaries. Estuaries 22, 313 326. freshwater in ow as an important source of orthophosphate to the Froelich, P.N., 1988. Kinetic control of dissolved phosphate in natural rivers and two estuaries, however, the spatial differences indicate ortho- estuaries: a primer on the phosphate buffer mechanism. Limnol. Oceanogr. 33, phosphate concentrations dropped quickly after entering estuaries. 649e668. fi Garcia-Luque, E., Pajares, J.M.F., Gomez-Parra, A., 2006. Assessing the geochemical In the lab, it was identi ed that the added orthophosphate con- reactivitity of inorganic phosphorus along estuaries by means of laboratory centrations get adsorbed into the sediment composition simulation experiments. Hydrol. Process. 20, 3555e3566. (calcite þ organic matter), which explains the low concentrations Gomez, E., Durrillon, C., Rofes, G., Picot, B., 1999. Phosphate adsorption and release fl found in the two estuaries. Higher orthophosphate concentration from sediments of brackish lagoons: pH, O2 and loading in uence. Water Res. 33, 2437e2447. in without organic treatments of the Guadalupe Estuary indicates Howarth, R.W., Jennesen, H.S., Marino, R., Postma, H., 1995. Transport to and pro- dissolution of calcite could maintain orthophosphate in the water. cessing of phosphorus in near-shore and oceanic water. In: Tiessen, H. (Ed.), In the treatments without sediment organic matter, the ortho- Phosphorus in the Global Environment: Transfers, Cycles and Management. John Wiley and Sons, Chichester, England, pp. 323e345. phosphate concentration was higher in the Guadalupe Estuary Ingall, E.D., Jahnke, R., 1997. Influence of water column anoxia on the elemental compared to the Nueces Estuary treatments. Calcite bonded fractionation of carbon and phosphorus during sediment diagenesis. Mar. Geol. phosphorus may be the reason for the variation of orthophosphate 139, 219e229. Krauskopf, K.B., 1956. Dissolution and precipitation of silica at low temperatures. concentrations in the estuaries. Hence, this research demonstrates Geochim. Cosmochim. Acta 10, 1e26. that interactions between physical forcing, salinity, sediment Krom, M.D., Berner, R.A., 1981. The diagenesis of phosphorus in near shore marine minerals, and organic matter can regulate silicate and orthophos- sediment. Geochim. Cosmochim. Acta 45, 207e216. Lebo, M.E., 1991. Particle-bound phosphorus along urbanized coastal plain estuary. phate concentrations in the estuaries. Mar. Chem. 34, 225e246. Liss, P.S., Spencer, C.P., 1970. Abiological processes in the removal of silicate from sea Acknowledgment water. Geochim. Cosmochim. Acta 34, 1073e1088. McGlathery, K.J., Marino, R., Howarth, R.W., 1994. Variable rates of phosphate uptake by shallow marine carbonate sediments: mechanisms and ecological sig- This work was supported in part by grant number nificance. Biogeochemistry 25, 127e146. NA09NMF4720179 from the National Oceanic and Atmospheric Montagna, P.A., Palmer, T.A., Pollack, J.B., 2013. Hydrological Changes and Estuarine Administration (NOAA) under the Comparative Assessment of Dynamics. http://dx.doi.org/10.1007/978-1-4614-5833-3. SpringerBriefs in Environmental Sciences, New York, New York. Marine Ecosystem (CAMEO) program, award NNX11AE42G from Morris, A.W., Bale, A.J., Howland, R.J.M., 1981. Nutrient distribution in an estuary: the National Aeronautics and Space Administration (NASA) via evidence of chemical precipitation of dissolved silicate and phosphate. Estuar. e subcontract UTA11-000400 from the University of Texas at Austin, Coast. Shelf Sci. 12, 205 216. Mortimer, C.H., 1942. The exchange of dissolved substances between mud and and the Harte Research Institute for Gulf of Mexico Studies Grad- water in lakes. J. Ecol. 30, 147e201. uate Research Fellowship. The authors would like to thank Thomas Naehr, T.H., Rodriguez, N.M., Bohrmann, G., Paull, C.K., Botz, R., 2000. Methane- Naehr for providing logistic support and for giving guidance in the derived authigenic carbonates associated with gas hydrate decomposition and fluid venting above the Blake Ridge Diapir. In: Paull, C.K., Matsumoto, R., mineral analysis. The authors would also like to thank Rick Kalke Wallace, P.J., Dillon, W.P. (Eds.), Proceedings of the Ocean Drilling Program, and Larry Hyde for their help in collecting sediment samples, and Scientific Results. Ocean Drilling Program, College Station, TX, pp. 285e300. 50 B. Paudel et al. / Estuarine, Coastal and Shelf Science 157 (2015) 42e50

Nielsen, K., Risgaard-Peterson, N., Somod, B., Rysgaard, S., Bergo, T., 2001. Nitrogen Staudinger, B., Peiffer, S., Avnimelech, Y., Berman, T., 1990. Phosphorus mobility in and phosphorus retention estimated independently by flux measurements and interstitial waters of sediments in Lake Kinneret, Israel. Hydrobiologia 207, dynamic modeling in the estuary, Randers Fjord, Denmark. Mar. Ecol. Prog. Ser. 167e177. 219, 25e40. Sundareshwar, P.V., Morris, J.T., 1999. Phosphorus sorption characteristics of inter- Paudel, B., 2014. Interaction Between Suspended Sediments, Nutrients and Fresh- tidal marsh sediments along an estuarine salinity gradient. Limnol. Oceanogr. water Inflow in Texas Estuaries. PhD Thesis. Texas A&M University, Corpus 44, 1693e1701. Christi, p. 210. Tanaka, M., Takahashi, K., 2005. Characterization of silicate complexes in aqueous Rickert, D., 2000. Dissolution kinetics of biogenic silica in marine environments. In: sodium sulfate solutions by FAB-MS. J. Solut. Chem. 34, 617e630. Reports on Polar Research, vol. 351. Alfred Wegener Institute for Polar and Tanaka, M., Takahashi, K., 2007. Study on the salting-out effect using silica species Marine Research, p. 211. by FAB-MS. J. Solut. Chem. 36, 27e37. Rickert, D., Schluter, M., Wallmann, K., 2002. Dissolution kinetics of biogenic silica Tanaka, M., Takahashi, K., 2013. Ionized silica in the estuary of a river as supply to from the water column to the sediments. Geochim. Cosmochim. Acta 66, seawater: identification and ionization efficiency of silica species by FAB-MS. 439e445. Estuar. Coast. Shelf Sci. 121e122, 1e7. Schumacher, B.A., 2002. Methods for the Determination of Total Organic Carbon Turner, E.L., 2014. Modeling Nutrient Dynamics in Coastal Lagoons. PhD Thesis. (TOC) in Soils and Sediments. USEPA Environment Science Division National Texas A&M University, Corpus Christi, p. 146. Exposure Research Laboratory. Upchurch, J.B., Edzwald, J.K., O'Melia, C.R., 1974. Phosphates in sediments of Pamlico Spagnoli, F., Bergamini, M.C., 1997. Water-sediment exchange of nutrients during Estuary. Environ. Sci. Technol. 8, 56e58. early diagenesis and resuspension of anoxic sediments from the Northern Williams, L.A., Crerar, D.A., 1985. Silica diagenesis, II. General mechamisms. Adriatic Sea shelf. Water, Air Soil Pollut. 99, 541e556. J. Sediment. Petrol. 55, 312e321.