Temporal and Spatial Dynamics of Particulate Organic Carbon in the Lake Pontchartrain Estuary, Southeast Louisiana, U.S.A

Estuarine, Coastal and Shelf Science (1997) 45, 557–569

Temporal and Spatial Dynamics of Particulate Organic Carbon in the Lake Pontchartrain Estuary, Southeast Louisiana, U.S.A. T. S. Bianchia and M. E. Argyroua,b a

Department of Ecology, Evolution, and Organismal Biology, Tulane University, New Orleans, Louisiana 70118, U.S.A. b Department of Fisheries, Ministry of Agriculture, Natural Resources and Environment, 13 Aeolou Street, Nicosia, Cyprus Received 15 October 1996 and accepted in revised form 28 January 1997 The spatial and temporal distribution of particulate organic carbon (POC) was examined in the Lake Pontchartrain estuary, along with changes in freshwater and nutrient inputs. Increased turbidity due to increases in suspended particulate matter (SPM) concentrations during high freshwater discharge resulted in low inputs from autochthonous POC. For example, annual averages of chlorophyll a (2·3&0·9 ìg l "1) and POC (0·9&0·6 mg l "1) were found to be considerably lower than in other shallow turbid systems. A low N/P ratio was maintained throughout the study (from 2·3 to 8·3), primarily due to low inputs of dissolved inorganic nitrogen (DIN). Thus, phytoplankton communities in the Lake Pontchartrain estuary were primarily limited by light and nitrogen. Stations in the upper estuary were generally more light-limited as opposed to nitrogen-limited stations in the lower estuary. Based on chlorophyll a/carotenoid ratios, the most dominant phytoplankton classes were cyanobacteria, cryptophytes, chrysophytes and chlorophytes—with episodic cyanobacterial blooms occurring in the summer months. High carbon/chlorophyll a ratios suggested that much of the POC contained low concentrations of chlorophyll a that had degraded during its transport from adjacent wetlands. High C/N ratios (7·4 to 30·6) further suggested that allochtonous inputs of organic carbon from terrigenous sources were likely to be important. Lignin-phenol concentrations in sediments from the upper region of the estuary indicated that the Lake Pontchartrain estuary had received higher inputs of terrigenous organic carbon compared to other estuaries in the Gulf region. ? 1997 Academic Press Limited Keywords: particulate organic carbon; plant pigments; nutrients; lignin-phenols

Introduction A diverse array of allochthonous and autochthonous POC sources (terrestrial, submersed macrophytes, plankton) enter into the food webs of riverine and estuarine ecosystems (Wetzel, 1984; Valiela, 1995). Temporal variations of these sources are expected to vary seasonally with maximum phytoplankton production in summer and peak allochthonous inputs in spring and winter or late fall. Understanding the patterns of cycling of these POC sources is crucial to understanding the trophic dynamics of land-margin ecosystems. The determination of photosynthetic pigments and their degradation products has proven to be a useful method for determining organic matter sources in aquatic systems (Mantoura & Llewellyn, 1983; Bianchi et al., 1993, 1995, 1997a). The decay and fate of these sources to the heterogeneous detritus pool is of considerable interest to understanding energy flow and production in aquatic ecosystems 0272–7714/97/050557+13 $25.00/0/ec970237

(Valiela, 1995). While carotenoids exhibit classspecificity that can be used as tracers for phytoplankton and algae, other chemical markers are needed in land-margin ecosystems to determine inputs of terrestrial vascular plants. Over the past two decades, lignin has been successfully used as a chemical biomarker for documenting inputs of vascular plant materials and represents an important source of sedimentary carbon in coastal areas (Hedges & Parker, 1976; Hedges & Ertel, 1982). Lignin is a macromolecule found in cell walls of vascular plants which, upon oxidation (via CuO oxidation), can yield eight dominant vanillyl, syringyl and cinnamyl phenols (Hedges & Parker, 1976; Hedges & Ertel, 1982). These lignin phenols can be used to distinguish between woody and non-woody tissues of gymnosperms and angiosperms. The syringyl derivatives are unique to woody and non-woody angiosperms, while the cinnamyl groups are common to non-woody angiosperms and gymnosperms. ? 1997 Academic Press Limited

558 T. S. Bianchi & M. E. Argyrou ncta Tchefu River

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F 1. Map of the Lake Pontchartrain estuary, showing the nine stations (St. 1, 4, 7, upper region; St. 2, 5, 8, middle region; St. 3, 6, 9, lower region), sampled from May 1995 to May 1996.

In shallow turbid estuaries, which tend to be well mixed and usually unstratified, gross and net primary production are primarily controlled by light availability to phytoplankton (Pennock, 1985; Cole et al., 1992). Shallow turbid estuaries are usually more productive since phytoplankton POC is governed by light availability along with high nutrients from riverine and terrestrial inputs, and from remineralization processes in sediments (Kremer & Nixon, 1978; Nixon, 1981; Kemp & Boynton, 1984). Highly variable light regimes typically found in shallow turbid estuaries have also been shown to stimulate phytoplankton productivity by reducing photoinhibition (Madden & Day, 1992; Mallin & Paerl, 1992). While these systems may have higher production, the residence time for this organic carbon is typically short because of enhanced freshwater inputs (Baskaran et al., 1997). This study investigated the sources and spatial/ temporal dynamics of POC in relation to nutrient cycling in the Lake Pontchartrain estuary. Riverine nutrient inputs were used to develop a conceptual model on nutrient budgets in this system. The study objectives were as follows: (1) to use photosynthetic pigments as chemical biomarkers to identify dominant sources of POC in the water column and sediments of the Lake Pontchartrain estuary; (2) to use ligninphenols as tracers of sources of terrestrially-derived

carbon in sediments at selected stations in the estuary; and (3) to determine the temporal and spatial distribution of water column nutrients in the estuary. Materials and methods Site description The Lake Pontchartrain estuary is located in the central Gulf Coastal Plain Province adjacent to New Orleans, Louisiana (Figure 1). The system is composed of two lakes, Pontchartrain (1645 km2, average depth of 4 m) and Maurepas (241 km2, average depth 2 m). The lakes are located in Holoceneage lacustrine, delta-lobe, channel-fill, and barrier island deposits (Flowers & Isphording, 1990). Lake Pontchartrain formed approximately 3000 years ago when the St. Bernard lobe of the Mississippi River delta closed off the Pontchartrain embayment, which formed during the sea-level rise after the Wisconsin glaciation (Kolb et al., 1975). Average riverine discharges into Lake Maurepas and Lake Pontchartrain were estimated to be 144 m3 s "1 and 228 m3 s "1, respectively (Flowers & Isphording, 1990). Additional discharge into Lake Pontchartrain occurs from stormwater drainage as well as freshwater diversion from the Mississippi through the Bonnet Carre´ spillway.

Temporal and spatial dynamics of particulate organic carbon 559

Sediment inflow to the estuary ranks among the lowest for any of the northern Gulf estuaries; however, the sediment volume discharged into the lake via the Bonnet Carre´ spillway can be as much as nine times the average annual sediment load (0·76 million tons years "1) (Isphording et al., 1989). Bottom sediments are typically dominated by clays and silts representing 43 and 38% of the total sediment fraction, respectively (Flowers & Isphording, 1990). Lake Pontchartrain is a highly productive estuarine ecosystem that is vital to many of the local commercially important fisheries in the Gulf of Mexico. While there have been some studies on the trophic dynamics of fishes in the estuary and its adjacent wetlands, such as Bayou La Branche and Trepagnier (Darnell, 1961; Hinchee, 1977), virtually nothing has been done with respect to nutrient inputs and overall carbon cycling in the sediments and water column in the lake. Sampling regime In this study, three transects were sampled (each with three stations), representing the upper, middle and lower estuarine regions, respectively (Figure 1). During the period from May 1995 until May 1996, water and sediment samples were collected aboard the RV Callinectes along all nine stations in the upper, middle and lower transects (Figure 1) on a bimonthly basis. In March 1996, water samples (for nutrient analyses) were also collected from all the substantial freshwater sources (Lake Maurepas, Manchac Pass, Amite River, Tangipahoa River, and Tchefuncta River) that drain into the Lake Pontchartrain estuary. Surface and bottom water samples were collected from each station using a peristaltic pump and stored frozen for SPM, particulate organic carbon (POC) and nitrogen (PON), plant pigments, and nutrient analyses. Sediment samples were also collected in duplicate from each station using a hand-held corer. The top 1 cm of each core was split for analysis of plant pigments, sedimentary organic carbon (SOC) and nitrogen (SON), percent organic matter and lignin phenols. Water column. Hydrographic data such as salinity, temperature, dissolved oxygen, pH, and photosynthetically active radiation (PAR) were recorded using YSI model 33 S-C-T meter, YSI model 51B oxygen meter, HI9025 microcomputer pH meter, and a LI-COR (LI-189) spherical/quantum/radiometer/ photometer, respectively. The light attenuation coefficient (k) was calculated, only for the months November 1995, January 1996, and March 1996,

using the Beers–Lambert expression (Parsons et al., 1979): Iz =Io (e "kz) where, Iz =light at any depth z, Io =light at the top of the water column, and k=diffuse downwelling irradiance attenuation coefficient. SPM, POC and PON analyses. Suspended particulate matter (SPM) and POC/PON were collected using a polycarbonate membrane filter (0·2 ìm pore size) and a pre-combusted (0·7 ìm) glass fibre filter, respectively. Following acid treatment (Bianchi et al., 1997a,b), filters were analysed for POC and PON using a Carlo Erba NA1500 NCS system. Plant pigment analysis. Particulates from surface and bottom waters were collected for pigment analyses by filtering water (c. 200 ml) through a GF/F glass fibre filter (25 mm diam., nominal pore size of 0·7 ìm). Plant pigments were extracted and analysed by high performance liquid chromatography (HPLC) according to the methods described by Wright et al., (1991), as modified by Bianchi et al. (1995). The method allowed for adequate resolution of dominant peaks of interest, as well as separation of the sterio-isomers lutein and zeaxanthin (Wright et al., 1991) (Figure 2). High purity HPLC standards for chlorophylls a and b and â,â-carotene were obtained from Sigma Co. Standards of the following carotenoids were kindly provided by Hoffman LaRoche Co., Basel, Switzerland: fucoxanthin; alloxanthin; zeaxanthin. Nutrient analysis. Surface and bottom water samples were filtered through pre-combusted (450 )C) GF/F Whatman filters (47 mm diam., nominal pore size of 0·7 ìm) and stored frozen in acid-rinsed polypropylene bottles prior to nutrient analysis (NH4+ , PO4"3, NO3" -NO2" ). All nutrient samples were analysed within 24–48 h of collection. Ammonium measurements were performed using the phenol-hypochlorite method described by Solorzano (1969). Phosphate, nitrite, and nitrate measurements were performed according to the spectrophotometric techniques of Strickland and Parsons (1972). Sediments Plant pigment, SOC, SON and organic matter analyses. Plant pigments were extracted from sediments following the same procedure previously described for water column samples. The only difference is that the HPLC gradient was slightly modified for sediments

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F 2. Reversed-phase HPLC absorbance chromatogram showing the separation and elution of chloropigments and carotenoids found in the water column of the Lake Pontchartrain estuary. The identified pigment peaks are as follows: 1, fucoxanthin; 2, alloxanthin; 3, zeaxanthin; 4, chlorophyll b; 5, chlorophyll a; 6, â-carotene.

resulting in a longer run time of 49 min. SOC and SON were obtained by the same procedure as POC and PON described earlier. Percent organic matter was measured by weight loss upon combustion at 450 )C for 16 h. Lignin-phenol analysis. Lignin-phenols were extracted from sediments using CuO oxidation and then analysed by capillary gas chromatography according to the method of Hedges and Ertel (1982), as modified by Bianchi et al. (1997b). Samples were analysed on a Shimadzu capillary GC Model GC-14 APSCF. Verification of lignin-phenols were performed using a Finnigan GC-MS. Statistical analyses. An Fmax was used prior to ANOVA to check for homogeneity of variances. Twoway ANOVA was used to test for significant effects within and among hydrographic data, SPM, plant pigments, nutrients, POC and river inflow (Sokal & Rohlf, 1995). When ANOVA differences were significant (i.e. P<0·05), a Scheffe´ multiple range test was performed to detect for differences (P<0·05) among stations and among different sampling dates. Spearman rank correlation analysis was also

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F 3. Monthly precipitation (cm) and total freshwater inflow (m3 s "1) from the Amite, Tickfaw, Tangipahoa, Tchefuncta Rivers and Bonnet Carre´ Spillway, into the Lake Pontchartrain estuary from May 1995 to May 1996.

performed to test for relationships among all the variables. Results Water column Throughout most of the study, there were no significant differences in nutrients (NH4+ , PO4"3, NO3" NO2" ) concentrations between stations. While this estuary is well mixed and does not appear to have pronounced spatial differences in many of the measured parameters, there is a significant gradient of freshwater flow (via rivers) into the estuary that generally moves from west to east. Based on this gradient of freshwater inflow, the nutrient data from individual stations have been combined and presented here as averaged regional data. The estuary was divided into upper (St. 1, 4, 7), middle (St. 2, 5, 8) and lower (St. 3, 6, 9) regions. Hydrographic data. Monthly precipitation varied from 2 to 55 cm, reaching its highest in May 1995 and its lowest in May 1996 (Figure 3). Total river discharge (Amite, Tickfaw, Tangipahoa and Tchefuncta Rivers) as monitored by the United States Geological Survey (USGS) was lowest in May 1996 (63 m3 s "1) and highest in May 1995 (276 m3 s "1) (Figure 3). The

Temporal and spatial dynamics of particulate organic carbon 561

Suspended particles and particulate organic carbon and nitrogen measurements. Concentrations of suspended particulate matter (SPM) in all three regions of the estuary ranged from 3 to 83 mg l "1, the highest (P<0·05) concentrations of SPM were observed in May 1995 (21·3&3·4 mg l "1), January 1996 (32·1&28·6 mg l "1), and March (28·5&9·8 mg l "1), all in the upper region of the estuary. SPM was significantly correlated with riverflow (P<0·05). POC ranged from 0·2 to 4·2 mg l "1; the highest concentration of POC (4·0&0·1 mg l "1) occurred in January 1996 in the upper region, when the average

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high river inflow in May 1995 (176 m3 s "1) was due to extensive flooding (100 year flood) that New Orleans experienced at that time. Extremely high freshwater discharges (291 m3 s "1) occurred at the end of June 1995, just after the flooding event, due to leakage of the Bonnet Carre´ Spillway (Figure 3). The inflow of water from Lake Maurepas, which also drains into the Lake Pontchartrain estuary via a tidal channel known as Manchac Pass, was assumed to be represented by river discharge into Lake Maurepas. However, since the Blind River was not monitored by the USGS, the inputs are likely to represent an underestimate of total river inflow. While there were some occasions when stratification was observed between bottom and surface waters, in most cases the system remained well mixed, thus all reported values will be based on averaged surface and bottom measurements. Water temperatures over the entire sampling period ranged from 10 to 31 )C across all three regions. No significant differences in temperature were observed between the three regions of the estuary (P>0·05). However, water temperatures were significantly different between sampling dates (P<0·05); the lowest average temperature (among the three regions) occurred in March 1996 (10·7&0·4 )C, n=18) and the highest in July 1995 (30·7&0·4 )C, n=18). Dissolved oxygen (DO) ranged from 2 to 13 mg l "1 among all three regions. There were no significant differences in DO between regions of the estuary; the highest (P<0·05) average concentration of DO in all regions of the estuary (9·4&1·2, n=18) occurred during March 1996. Average salinities ranged from below detection to 8 throughout the three regions (data not shown). Average salinity was significantly different among the three regions of the estuary (P<0·05); the upper region was found to have significantly lower salinity than the mid-region which had significantly lower salinity than the lower region (P<0·05). The highest average salinity occurred in September 1995 (4·7&1·7, n=18) during the lowest inflow period.

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F 4. Percent particulate organic carbon (POC %) and nitrogen (PON %) of total suspended particulate matter (SPM) vs concentration of SPM (mg l "1) in the water column from all three regions of the Lake Pontchartrain estuary, sampled from May 1995 to May 1996.

regional concentration of chlorophyll a was at its lowest (1·3 ìg l "1). There was a significant negative correlation (R= "0·43, n=108, P<0·05) between POC and SPM, in fact, as SPM concentrations decreased below 10–20 mg l "1, the %OC content of the particles increased sharply (Figure 4). PON concentrations ranged from 9 to 326 ìg l "1 for all three regions, with the highest concentration occurring in January 1996. PON was negatively correlated with SPM (R= "0·46, n=108, P<0·05) (Figure 4) and was correlated with POC (R=0·80, n=108, P<0·05) (data not shown). The atomic carbon/nitrogen (C/N) ratio of particulate organic matter (POM) ranged from 7·4 to 30·6, with the highest C/N ratio observed in November 1995 and January 1996 (Figure 5). This C/N ratio is similar to what is typically found in freshwater systems (Wetzel, 1983). Light attenuation coefficients typically ranged from 0·6 to 3·7 m "1; the highest light attenuation observed in the upper and middle regions of the estuary was in January 1996, when SPM concentrations were at their highest (Figure 5).

562 T. S. Bianchi & M. E. Argyrou

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F 5. Carbon nitrogen (C/N) ratios of particulate organic carbon (POC) and nitrogen (PON) for all nine stations of the estuary, from May 1995 to May 1996, and light extinction coefficients (k) (m "1) for the months of November 1995, January 1996, and March 1996. U, upper region (St. 1, 4, 7); M, middle region (St. 2, 5, 8); L, lower region (St. 3, 6, 9).

Plant pigment measurements in POC. Chlorophyll a concentrations ranged from 0·3 to 6·9 ìg l "1 (Figure 6). The highest (P<0·05) chlorophyll a concentrations (6·0&0·9 ìg l "1) and (6·9&1·5 ìg l "1) occurred in July 1995 at Station 8 and in March 1996 at Station 4, respectively. Conversely, the lowest chlorophyll a concentration (0·3&0·1 ìg l "1) occurred in January 1996 at Station 1—where freshwater input and SPM were high. Chlorophyll a concentrations were not significantly correlated (P<0·05) with either POC or PON concentrations (data not shown). The ratios of POC/chlorophyll a ranged from 93 to 1608; the highest ratio was observed in January 1996 when SPM and POC concentrations were at their highest. Chlorophyll a concentrations were not correlated with ammonium and phosphate concentrations (data not shown). Concentrations of the accessory pigment chlorophyll b (a marker for chlorophytes) were significantly correlated (R=0·45, n=160, P<0·05) with chlorophyll a concentrations for the entire sampling

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F 6. Water column concentrations (ìg l "1) of chlorophyll a and chlorophyll b for all nine stations of the Lake Pontchartrain estuary, sampled from May 1995 to May 1996.

period at all stations. Chlorophyll b concentrations ranged from below limits of detection (BLD) to 1·1 ìg l "1 (Figure 6). The highest concentrations (P<0·05) of chlorophyll b were observed in July 1995 at Station 3 (1·14&0·04 ìg l "1) and in November 1995 at Station 4 (1·13&0·03 ìg l "1). Carotenoids, which are more class-specific than chlorophylls, were used to determine the dominant phytoplankton classes throughout the study. Concentrations of zeaxanthin, a carotenoid found in cyanobacteria, ranged from 0·03 to 2·1 ìg l "1 (Figure 7). No significant differences (P>0·05) in zeaxanthin concentrations were observed among all nine stations of the estuary. The highest concentrations (P<0·05) of zeaxanthin occurred in May 1995 (1·1&0·1 ìg l "1), July 1995 (2·1&0·2 ìg l "1) and September 1995 (1·20&0·02 ìg l "1). Zeaxanthin concentrations were significantly correlated (R=0·56, n=160, P<0·05) with chlorophyll a concentrations. Fucoxanthin (a biomarker for diatoms) ranged from BLD to 1·2 ìg l "1 (Figure 7); the highest concentrations of fucoxanthin occurred in May 1995, July 1995 and March 1996, at Station 4 (P<0·05).

Temporal and spatial dynamics of particulate organic carbon 563

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F 7. Water column concentrations (ìg l "1) of zeaxanthin, fucoxanthin and alloxanthin for all nine stations of the Lake Pontchartrain estuary, sampled from May 1995 to May 1996.

Fucoxanthin concentrations were significantly higher in March 1996 (P<0·05); concentrations of fucoxanthin were correlated (R=0·65, n=160, P<0·05) with chlorophyll a concentrations. Alloxanthin, a marker for cryptophytes, ranged from BLD to 0·7 ìg l "1 (Figure 7), the highest alloxanthin concentrations occurred in March 1996 (0·7&0·1 ìg l "1) at Station 4. To determine the relative percentage of different classes of phytoplankton, the following pigment ratios

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F 8. Percent abundances of dominant phytoplankton classes in the water column of the Lake Pontchartrain estuary, from May 1995 to March 1996.

from pure cultures were used: chlorophyll a/ chlorophyll b ratio of 0·7 (chlorophyceae), chlorophyll a/fucoxanthin ratio of 1·4 (chrysophyceae), chlorophyll a/zeaxanthin of 1·7 (cyanophyceae) and chlorophyll a/alloxanthin of 2·0 (cryptophyceae) (Everitt et al., 1990). Throughout the entire sampling period, it was estimated that 4–13% of the total chlorophyll a pool was comprised of chlorophytes, 3–22% was comprised of chrysophytes, 8–62% was comprised of cyanophytes, and 8–22% was comprised of cryptophytes (Figure 8). The highest percent

564 T. S. Bianchi & M. E. Argyrou

Sediments Carbon, chlorophyll a, and lignin-phenol measurements. Percent organic matter ranged from 1·1 to 5·9% (data not shown). The highest average percent organic matter (P<0·05) was observed at Station 1, while the lowest occurred at Station 9. Percent organic matter was significantly correlated with organic carbon (R=0·83, n=90, P<0·05) and organic nitrogen (R=0·83, n=90, P<0·05; R=0·43, n=90, P<0·05) (Figure 11). Chlorophyll a concentrations in sediments ranged from BLD to 0·3 ìg g dry sed "1 at all stations at both sampling dates (March 1995 and July 1995) (data not shown). Total lignin-derived phenols

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Nutrient measurements. The range of regional (i.e. upper, middle and lower) nutrient concentrations (PO4"3, NH4+ , and NO3" -NO2" ) throughout the estuary were as follows: PO4"3 concentrations ranged from 1·2 to 2·0 ìM; NH4 + concentrations ranged from 3·2 to 8·5 ìM; combined nitrate and nitrite concentrations ranged from 0·1 to 7·5 ìM (Figure 9). There were no significant differences in PO4"3 concentrations between the three regions of the estuary (P>0·05) over the entire sampling period. Regional NH +4 concentrations were significantly (P<0·05) higher in January 1996 (8·5&2·6 ìM) and March 1996 (8·0&3·8 ìM) in the upper region. Concentrations of NO3" and NO2" were generally low except during periods of high river inflow and were significantly correlated with total river discharge (R=0·58, n=162, P<0·05). The highest concentrations of NO3" and NO2" were observed in the upper region in January 1996 (6·2&3·1 ìM) and in March 1996 (7·5&3·3 ìM). The nitrogen/phosphorus (N/P) ratio which ranged from 2·3 to 8·3 was considerably lower than the expected Redfield ratio (N/P=16). The highest N/P ratio (P<0·05) was observed in March 1996 (8·3&1·9) in the upper region (Figure 10).

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abundance of chlorophytes (13%) and cyanophytes (62%) of the total chlorophyll a were observed in September 1995. Chrysophytes (22%) and cryptophytes (22%) reached their highest percentage in May 1995 and July 1995. A fraction of the total chlorophyll a pool during November 1995 (59%), January 1996 (55%), and March 1996 (43%) appeared to have a non-planktonic origin (Figure 8). However, it should be noted that while the non-planktonic percentages appear to be quite high, the chlorophyll a concentrations were very low during these months (i.e. 1·2 to 3·9 ìg l "1).

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F 9. Water column nutrients concentrations (ìM) from all three regions of the Lake Pontchartrain estuary, sampled from May 1995 to March 1996.

ranged from 8·5 to 31 mg 100 mg sed "1 (Figure 12) in the upper region of the estuary. The highest ligninphenol concentrations were observed at Station 1. Discussion Importance of freshwater inputs on the distribution and composition of POC Seasonal variations in freshwater discharge to the Lake Pontchartrain estuary contributed significantly to the spatial and temporal distribution of POC. The highest observed POC and SPM concentrations occurred during winter and early summer, when freshwater discharges were at their maximum. Although

Temporal and spatial dynamics of particulate organic carbon 565

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1996

F 10. Nitrogen/phosphorus ratios for all three regions of the Lake Pontchartrain estuary, sampled from May 1995 to March 1996.

POC values were higher during January 1996, these particles contained less organic carbon per weight percent of total SPM, than observed during the summer months. POC concentrations, which ranged from 0·2 to 4·2 mg l "1, were considerably lower than other shallow river-dominated estuaries (Boynton et al., 1982). However, as SPM concentrations decreased below 10–20 mg l "1, the POC and PON content of the particles increased (Figure 4). This relationships was also found in another northern Gulf coast estuary, the Sabine-Neches estuary, which has similar SPM and POC concentrations (Bianchi et al., 1997a). In general, this trend is consistent with patterns observed in larger rivers (Meybeck, 1982). In these highly turbid systems, the increase of %POC and %PON typically occurs at SPM concentrations of 50 mg l "1 (Meybeck, 1982; Milliman et al., 1984; Trefry et al., 1994). This relationship can be attributed to a dilution effect, due to an increase in sediment load at high freshwater discharges. Another important mechanism controlling SPM concentrations in this shallow system is resuspension of bottom sediments. It has been shown that sediments in Lake Pontchartrain estuary are capable of being resuspended by fairly low wind speeds (~10 mph) (Swenson, 1980).

Sedimentary organic nitrogen (%)

Nitrogen/phoshorus ratio

Sedimentary organic carbon (%)

12

2

4 Organic matter (%)

6

8

2

4 Organic matter (%)

6

8

0.4

0.2

0

F 11. Percent organic carbon (OC %) and nitrogen (ON %) vs percent organic matter (OM %) in the sediments from all nine stations of the Lake Pontchartrain estuary, sampled from May 1995 to May 1996.

The phytoplankton community in the Lake Pontchartrain estuary frequently experiences light limitation due to high SPM concentrations. The range of light extinction coefficients found in the Lake Pontchartrain estuary (c. 0·6 to 3·7 m "1) was similar to those found in other shallow turbid systems, such as the Hudson River and Sabine-Neches estuaries (Cole et al., 1992; Bianchi et al., 1997a). The light extinction coefficient in the Lake Pontchartrain estuary increased to a maximum of 3·7 m "1 during winter, when high river inflow and SPM concentrations occurred. It was during this time of the year that the lowest average regional chlorophyll a concentration (1·3&0·6 ìg l "1) was observed; the highest chlorophyll a concentration was observed in July 1995 (3·3&1·7 ìg l "1) when light extinction coefficients were generally <1 m "1. Cole et al. (1992) showed that gross and net primary production of phytoplankton in the Hudson River estuary was severely light limited and that cells could spend from 18 to 22 h day "1 below the 1% light level. Conversely, in other shallow turbid estuaries highly variable light regimes have been shown to actually stimulate phytoplankton

566 T. S. Bianchi & M. E. Argyrou 35

–1

Σ of V + S 100 mg sed

Lignin-phenols

30

NH4+ 6.1 × 108 kg yr–1

–1

Σ of V + S + C 100 mg sed

25 20 15

PO43– 2.3 × 108 kg yr–1

NO3– NO2– 6.2 × 108 kg yr–1

Riverine inputs

10 5

St .6

St .2

A ug .

St .1

A ug .

A ug .

St .7 ay

St .4 M

ay M

Pontchartrain

+

NH4 8 4.5 × 10 kg

NO3– NO2– 1.7 × 108 kg

Sabine-Neches 3–

5 4

PO4 3.1 × 108 kg

3

Lake Pontchartrain Estuary

2

F 13. Diagram showing the annual riverine loadings of nutrients (PO4"3, NH4+ , NO3" -NO2" ) to the Lake Pontchartrain estuary.

1

* ty ni

bi Sa

ha

T 1. Nutrient concentrations of dominant rivers that drain into the Lake Pontchartrain and Lake Maurepas estuarine systems in March 1996. Manchac Passage which connects these two estuaries is included for comparison

A

tc

Tr i

ne

* la fa

bo re Te r

h ut So

ya

* ne

ss Pa

tr ar ch nt Po

*

n

0

ai

Σ of P, V, and S Phenols (mg) 10 g sed–1

M

ay

St .1

0

* = from Hedges and Parker (1976)

F 12. Lignin-phenols concentrations in sediments from the upper region of the Lake Pontchartrain estuary, sampled during September 1995; comparisons are made with lignin-phenol concentrations in sediments collected from other estuaries in the Gulf region in a study by Hedges and Parker (1976).

production (Mallin & Paerl, 1992; Madden & Day, 1992). Based on the low N/P ratios in the Lake Pontchartrain estuary, it appeared that phytoplankton were limited by nitrogen. Phytoplankton are generally considered to be nitrogen-limited if the N/P ratio is less than or equal to 10; from a regional perspective N and Si limitation have been shown in the Mississippi Plume (Dortch & Whitledge, 1992). The N/P ratios, which ranged from 2·3 to 8·3 (Figure 10), were positively correlated with freshwater discharge into the estuary. Stations in the upper-estuary were generally more light-limited as opposed to nitrogen-limited stations in the lower-estuary. The wide range of N/P ratios were primarily due to changes in dissolved inorganic nitrogen (DIN) species (NH4+ and NO3" NO2" ), since PO4"3 concentrations remained stable throughout the year (Figure 9). Moreover, DIN and

Rivers Tchefuncta Tangipahoa Amite Manchac Pass

PO4"3 (ìM)

NH4+ (ìM)

NO3" -NO2" (ìM)

N/P

1·9 1·7 2·4 1·8

15·9 9·6 15·2 7·8

8·7 17·4 10·7 12·1

12·4 15·9 10·9 11·2

N/P, nitrogen/phosphorus ratio.

N/P ratios in this system were considerably lower than those found in riverine discharges (for March 1996) (Table 1). To further understand the dynamics of nutrients in this specific environment, the authors attempted to develop a conceptual model on nutrient budgets, taking into consideration the average annual discharge of the rivers to the Lake Pontchartrain estuary (142 m3 s "1), the surface area (1645 km2), and total volume (6580#106 m3) of the estuary (Figure 13). total riverine inputs of DIN (12·3#108 kg year "1) (in March 1996) were 81% higher than PO4"3 inputs (2·3#108 kg year "1) (Figure 13). However, the total DIN pool in the estuary (6·2#108 kg) was only 50% greater than the PO4"3 pool (3·1#108 kg). Nitrates/ nitrites appeared to be the most depleted form of DIN

Temporal and spatial dynamics of particulate organic carbon 567

in the estuary relative to total riverine inputs (Figure 13). The relatively stable and high concentrations of PO4"3 in the water column may have resulted from regeneration of phosphate by sediments in the Lake Pontchartrain estuary. Nixon et al. (1980) have shown that regeneration from sediments in Narragansett Bay can provide enough phosphate to support 50% of the primary production in the water column. Although phytoplankton appeared to be limited by light and nitrogen, the uptake of DIN in this system is likely due to both phytoplankton and bacteria. High phosphate and ammonia accompanied by low nitrate concentrations has been shown in some cases to be indicative of sewage inputs (Valiela, 1995). Sewage inputs to the Lake Pontchartrain have posed a serious problem in recent years (i.e. high coliform counts) with inputs from agricultural sources in the northern region (via Tchefuncta and Tangipahoa Rivers) and sewage runoff from New Orleans in the southern region. It has also been suggested that bacterial consumption of DOC is largely responsible for CO2 saturation in the waters of the Lake Pontchartrain estuary (Argryrou et al., 1997). The primary resource for bacteria in blackwater rivers in the southeastern U.S. has also been shown to be primarily derived from allochthonous resources (Wainright et al., 1992). Finally, this work supports previous studies that have demonstrated that light-limited turbid estuaries are dominated by heterotrophic processes which are supported by allochthonous resources (Findlay et al., 1991; Cole, 1992; Bianchi et al., 1997a). Compositional changes in the phytoplankton community were not correlated with changes in nutrient concentrations. Based on plant pigment ratios of chlorophyll a/zeaxanthin (1·7), chlorophyll a/ alloxanthin (2·01), chlorophyll a/fucoxanthin (1·4), and chlorophyll a/chlorophyll b (0·75) (from pure algal cultures) (Everitt et al., 1990), it was estimated that the dominant classes of phytoplankton in the estuary were cyanobacteria (36%), cryptophytes (13%), chrysophytes (11%), and chlorophytes (9%), respectively. The dominance of cyanobacteria in this nitrogen-limited system is likely due to their nitrogenfixing capabilities (Li et al., 1983; Everitt et al., 1990). The average chlorophyll a concentration in the Lake Pontchartrain estuary was relatively low (2·3&0·9 ìg l "1) throughout the study. The most significant changes in phytoplankton composition occurred in July 1995 during a cyanobacterial bloom (Anabaena sp.). Although the highest measured chlorophyll a concentration (6·9 ìg l "1) in July 1995 was not reflective of concentrations typically associated with ‘ true ’ bloom conditions (i.e. 80 ìg l "1 or greater)— our measurements were made at the late stages of the

bloom. Direct cell counts of cyanobacteria during the bloom were at 106 cells l "1—clearly within the range of bloom conditions (Quay Dortch, pers. comm.). These blooms which typically occur in June and July were previously reported by Dow and Turner (1980), who attributed this phenomenon to freshwater inputs from the Tchefuncta River and Manchac Pass. Although, leakage from the Bonnet Carre´ Spillway caused a dramatic increase in the freshwater discharge into the system during the 1995 bloom, no significant changes in nutrient concentrations were observed. Another explanation for this is that excess nutrients released from the Spillway may have already been taken up into phytoplankton biomass. In any event, the causes for these episodic nuisance blooms in the Lake Pontchartrain estuary remain uncertain and require further study. High C/N and carbon/chlorophyll a (C/Chl a) ratios in the upper and mid-regions of this estuary suggested that terrigenous inputs of organic carbon from rivers and surrounding wetlands are likely to be important. The regional range of C/N ratios was between 7·4 and 30·6 (Figure 5). These ratios were similar to what is typically found in freshwater systems which typically results from a mixing of organic matter derived from allochthonous (C/N=45·0) and autochthonous (C/N=6·0) sources (Wetzel, 1983). While it is well established that vascular plant inputs are characterized by high C/N ratios, preferential uptake of nitrogen by bacteria may have also contributed to these high C/N ratios. However, microscopic examination of filtered materials revealed the presence of significant quantities of fibrous fragments that were clearly derived from vascular plants. High C/Chl a ratios were also observed throughout the estuary (data not shown). In fact, the highest average ratio of all three regions (1608&942) was observed in January 1996, when SPM concentrations and riverine discharge were at their highest. The extremely high C/Chl a ratio at this time was primarily due to low chlorophyll a concentrations. During this low chlorophyll a period in January 1996, phytoplankton comprised 45% of the total chlorophyll a, while the other 55% was mostly derived from inputs of degraded vascular plant detritus (Figure 8). High C/Chl a ratios further suggests that allochthonous inputs of organic carbon (i.e. riverine input, wetlands runoff) are likely to have been substantial since the C/Chl a ratio of estuarine phytoplankton is typically 60·0 (Tantichodok, 1989). Concentrations of lignin-phenols (ë=0·39&0·07 and Ë=0·57&0·14) in a limited number of POC samples (stations 4 and 5) collected in January 1996 (Argyrou, 1996) were similar to those found in sediment trap materials from

568 T. S. Bianchi & M. E. Argyrou

Dabob Bay, Washington (Hedges et al., 1988). This similarity suggests that terrestrial inputs are important in the Lake Pontchartrain system. Moreover, ligninphenol concentrations in sediments from the upper region of the estuary indicated that the Lake Pontchartrain estuary has received higher inputs of vascular plant materials in comparison to other regional estuaries (Figure 12). These findings also support earlier work which suggested that the dominant consumer species in Lake Pontchartrain are principally supported by organic detritus primarily derived from allochthonous sources (Darnell, 1961). Conclusions On the basis of this field study in the Lake Pontchartrain estuary, it is concluded that: (1) phytoplankton are limited by light and nitrogen—with nitrates/nitrites as the most limiting form of DIN. (2) Based on plant pigment biomarkers, the dominant phytoplankton throughout most of the year are represented by Chlorophycae and Chrysophyceae; with Cyanophyceae dominating during bloom events in summer months. (3) Based on lignin-phenol concentrations, there are significant inputs of terrestrial organic matter to this system. (4) Freshwater inputs were an important controlling variable in determining the sources and transport of POC in the Lake Pontchartrain estuary. Acknowledgements The authors would like to thank the captain, Dan Llewellyn, of the RV Callinectes (owned and operated by Southeastern Louisiana University), for his assistance in the collection of field samples. The authors are especially grateful to Corey Lambert for his invaluable help with field sampling, lignin analyses, and for reading earlier drafts of the manuscript. The authors also thank Amy Bennett for her comments on earlier drafts of the manuscript. This research was funded by a grant to M. E. Argyrou from World Bank and UNESCO. References Argyrou, M. E. 1996 Spatial and Temporal Variability of Particulate and Dissolved Organic Carbon in the Lake Pontchartrain Estuary: the Use of Chemical Biomarkers. Master’s thesis, Tulane University (pp. 89). Argyrou, M. E., Bianchi, T. S. & Lambert, C. D. 1997 Transport and fate of particulate and dissolved organic carbon in the Lake Pontchartrain estuary, Louisiana, U.S.A. Biogeochemistry 38, 207–226.

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