Control of Phytoplankton Growth: Nitrogen, Phosphorus, or Grazers? •David Wong1, Nancy N Rabalais2,3, R. Eugene Turner3, Ling Ren4 •1.Massachusetts Department of Environmental Protection •2.Louisiana Universities Marine Consortium •3.Louisiana State University •4.George Mason University Primary production (P): P Growth t Mortality Advectionx, y,z Diffusionx, y,z Sinking •Frost 1991 Phytoplankton Growth Control in Lakes • Bottom-up Control: Nutrient Limitation on Phytoplankton Growth • Top-down Control: Microzooplankton, mesozooplankton, and Bivalves Upper •New Orleans Barataria Basin •Davis Pond Diversion •Lac des Allemands •Lake •Cataouatche •Lake Salvador Phytoplankton Growth Control • Bottom-up Control: Nutrient Limitation on Phytoplankton Growth • Top-down Control: Microzooplankton, mesozooplankton, and Bivalves Experimental Lakes • Lac des Allemands • Lake Salvador • Lake Cataouatche •Wong et al. 2016 The physical conditions during the experiments: ambient lake salinity and Secchi disk depth, experiment duration •Ren et al. 2009 Nutrient Limitation Experiments • +N • +P, • +Si • +N+P • +N+Si • +P+Si • +N+P+Si • Control Nutrient Limitation Experiments • Nitrite+Nitrate • Phosphate • Ammonium • Silicate • Chl a • Phytoplankton Identification • Phytoplankton Enumeration • Suspended Particulate Matter • Samples taken at Time 0h, 6h, 12h, • Temp and 24h, • Solar Radiation • Every 24h after the first day and ended when Chl a started to • Light Intensity decrease The initial concentration of chlorophyll a (Chla), SPM, nutrients in lake waters, and the added nutrients in the bioassay experiments •Ren et al. 2009 Composition of phytoplankton (biovolume %) •Ren et al. 2009 Lake Cataouatche Lake Salvador Lac des Allemands The changes in Chl a concentration in response to N additions were different for all experiments •Ren et al. 2009 Five bioassays with N additions ( N, N + P, N + Si, and N+P+Si) • The concentration of Chl a increased significantly after N additions (i.e., after adding N, N + P, N + Si, and N+P+Si) • The addition of +Si,+P, and Si+P did not result in an increase in Chl a • N was the only limiting nutrient for phytoplankton growth in these five experiments Lake Cataouatche Lake Salvador Lac des Allemands The changes in Chl a concentration in response to N+P but More strongly By N than P Because +P Alone did not stimulate Additional growth •Ren et al. 2009 Lake Cataouatche Lake Salvador Lac des Allemands The addition of P Only resulted in marginally elevated Chl a concentration (P = 0.03) but was much less than N Additions (P < 0.001) •Ren et al. 2009 Chl a responses to nutrient additions and the results of nutrient limitation from bioassays +N > +P > +Si +N > +P > +N+P +N+P > Lakes Date Control Control >Control +P +N >+N +P Limitation Cataouatche 3-Oct y – – y – – y N 4-Jan y – – y – y y N+P 4-Apr y – – y – y y N+P 4-Jul y – – y – y y N+P Salvador 3-Oct y – – y – – y N 4-Jan y P=0.03 – y – – y N,P 4-Apr y – – y – y y N+P 4-Jul y – – y – – y N des Allemands 3-Oct y – – y – – y N 4-Jan y P=0.03 – y – – y N,P 4-Apr y – – y – y y N+P 4-Jul y – – y – – y N •Ren et al. 2009 Chl a responses to nutrient additions and the results of nutrient limitation from bioassays Nutrient Lakes deficiency Ndef Pdef Ndef/Pdef Cataouatche 6.9 0.6 12 4.9 3.2 1.5 3.5 0.9 3.5 5.2 3.2 1.6 Salvador 6.2 0.4 16 0.6 0.2 3 6.1 5.1 1.2 5.6 0.04 140 des •Ren et al. 2009 Allemands 1.3 0.02 65 0.2 0.04 5 1.1 0.3 3.4 0.9 0.07 12 The Nutrient Limitation Experiments • The results of these bioassays show that the phytoplankton growth in the lakes in upper Barataria Basin are primarily N- limited • P colimitation occurs - when it does, P is a secondary limiting nutrient Phytoplankton Growth Control • Bottom-up Control: Nutrient Limitation on Phytoplankton Growth • Top-down Control: Microzooplankton, mesozooplankton, and Bivalves Grazers Flagellates Ciliates Pelagic Community Rotifers Copepods Cladocerans Meroplankton larvae Bivalve molluscs Sponges Benthic Community Corals Cnidarians Tunicates Top-Down Control by Grazers • Microzooplankton • Mesozooplankton • Benthic grazer: Rangia clam Microzooplankton (< 200 µm) Microzooplankton Grazing Experiment Dilution Method (Landry et al. 1998) 202 m 0.2 m P = P e ( - m)t screened filtered t 0 water water - m = 1/t Ln (Pt / P0) 100 % 0% 1/t Ln (Pt / P0) = - m D + Dilution 100% 0% D: Dilution factor 50% 50% m: Microzooplankton Incubation 25% 75% grazing : Phytoplankton growth 12.5% 87.5% rate 6.25% 93.75% Lake Cataouatche Season Phytoplankton size Line R N P value Grazing rate (day -1) Fall 2003 < 5 micro y = -0.33x + 0.72 0.69 24 <0.01 0.33 5-20 micro * 0 > 20 micro * 0 Total phytoplankton y = -0.07x + 0.88 0.44 24 <0.01 0.07 Winter 2004 < 5 micro y = -0.57x + 0.35 0.77 24 <0.01 0.57 5-20 micro * 0 > 20 micro * 0 Total phytoplankton y = -0.22x + 0.34 0.84 24 <0.01 0.23 Spring 2004 < 5 micro ** 0.23 5-20 micro ** 0.67 > 20 micro * 0 Total phytoplankton * 0 Summer 2004 < 5 micro * 0 5-20 micro y = -0.90x + 1.84 0.84 24 <0.01 0.91 > 20 micro * 0 Total phytoplankton ** 0.04 Wong et al. 2016 Lake Salvador Season Phytoplankton size Line R N P value Grazing rate (day -1) Fall 2003 < 5 micro y = -0.85x + 0.85 0.87 24 <0.01 0.85 5-20 micro y = -0.34x + 1.02 0.74 24 <0.01 0.34 > 20 micro y = -0.19x + 0.10 0.44 24 <0.01 0.19 Total phytoplankton y = -0.63x + 0.89 0.89 24 <0.01 0.63 Winter 2004 < 5 micro y = -0.74x + 0.30 0.84 24 <0.01 0.74 5-20 micro * 0 > 20 micro y = -0.18x + 0.43 0.55 24 <0.01 0.18 Total phytoplankton y = -0.16x + 0.26 0.69 24 <0.01 0.16 Spring 2004 < 5 micro y = -0.46x + 0.47 0.52 24 <0.01 0.46 5-20 micro * 0 > 20 micro y = -0.64x + 0.89 0.54 24 <0.01 0.64 Total phytoplankton y = -0.44x + 0.97 0.85 24 <0.01 0.44 Summer 2004 < 5 micro y = -0.56x + 1.30 0.95 24 <0.01 0.56 5-20 micro y = -0.37x + 1.63 0.81 24 <0.01 0.37 > 20 micro y = -0.44x + 1.39 0.73 24 <0.01 0.44 Total phytoplankton y = -0.47x + 1.39 0.95 24 <0.01 0.47 Wong et al. 2016 Lac des Allemands Season Phytoplankton size Line R N P value Grazing rate (day -1) Fall 2003 < 5 micro y = -0.18x + 0.74 0.49 24 <0.01 0.18 5-20 micro y = -0.36x + 0.85 0.85 24 <0.01 0.36 > 20 micro y = -0.37x + 0.73 0.54 24 <0.01 0.37 Total phytoplankton y = -0.23x + 0.75 0.87 24 <0.01 0.23 Winter 2004 < 5 micro * 0 5-20 micro * 0 > 20 micro * 0 Total phytoplankton * 0 Spring 2004 < 5 micro y = -0.31x + 0.15 0.84 24 <0.01 0.31 5-20 micro * 0 > 20 micro * 0 Total phytoplankton y = -0.14x + 0.12 0.75 24 <0.01 0.14 Summer 2004 < 5 micro * 0 5-20 micro * 0 > 20 micro * 0 Total phytoplankton * 0 Wong et al. 2016 Microzooplankton Grazing Rate ) 0.5 -1 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 Microzooplankton grazing rate (day rate grazing Microzooplankton 0 < 5 micro 5 - 20 micro > 20 micro Wong et al. 2016 •0.5 •Instantaneous growth rate • Apparent growth rate •0.4 •0.3 c ) • 1 - • •0.2 b •0.1 •0 Growth rate (day rate Growth • •-0.1 < 5 m •-0.2 5 - 20 m •-0.3 > 20 m a •-0.4 Phytoplankton size composition 80 70 60 < 5 micro 50 40 5-20 micro 30 20 Percentage (%) Percentage 10 > 20 micro 0 0 50 100 150 200 Chlorophyll a (microgram l -1) Wong et al. 2016 Selective grazing by microzooplankton (Hansen et al. 1994) The size ratio between planktonic predators (Rotifers between 50 to 200 micro) and their preys (Phytoplankton) 18 : 1 Phytoplankton between 2.5 to 12 micro will be mostly consumed < 5 micro and 5 – 20 micro were impacted > 20 micro no significant impact Grazing on phytoplankton 1.0 < 5 m 0.8 Y = 0.44 X + 0.25 ) 1 - 5-20 m 0.6 Y = 0.53 X + 0.05 0.4 > 20 m Not Significant 0.2 Grazing rate (d rateGrazing Total phytoplankton 0.0 Y = 0.29 X + 0.14 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Instantaneous phytoplankton growth rate (d-1) Mesozooplankton (> 200µm) Mesozooplankton Grazing 1.8 2.5 ) ) < 5 micro -1 -1 1.6 Plot 1 Regr 5-20 micro 2.0 1.4 > 20 micro Total phytoplankton 1.2 1.5 1.0 0.8 1.0 0.6 0.5 0.4 0.2 0.0 0.0 Phytoplankton apparent growth rate (d rate growthapparent Phytoplankton Phytoplankton apparentgrowth rate (d -0.2 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Zooplankton density (x Natural density) Zooplankton density (x Natural density) Lac des Allemands Lake Salvador Wong et al.