20240717-002: Reservoir Update — original pdf

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The Reservoir Update – FY 2023 B r e n t B e l l i n g e r, P h . D . C o n s e r v a t i o n P r o g r a m S u p e r v i s o r Wa t e r s h e d P r o t e c t i o n D e p a r t m e n t J u l y 1 7 t h, 2 0 2 4 C I T Y O F A U S T I N W A T E R S H E D P R O T E C T I O N D E P A R T M E N T Projects • H o t b u t t o n t o p i c s • Aquatic vegetation – Lake Austin and Lady Bird Lake • O n - g o i n g • Reservoir monitoring • Harmful Algal Proliferations (HAPs; toxic cyanobacteria) • Sediment nutrient mitigation (lanthanum modified bentonite) • P ro j e c t s w r a p p e d u p ( i n p ro c e s s o f a n a l y z i n g / w r i t i n g ) Sediment bedforms and microplastics Zebra mussels population study • HAP monitoring at spring sites in collaboration with USGS • • • Herbivore exclosure pens • Lake Austin residential shoreline mitigation plantings C I T Y O F A U S T I N W A T E R S H E D P R O T E C T I O N D E P A R T M E N T 2 Austin Lakes Index R eservoirs maintaining “fair-to - good” condition System stressors: • Development of Lake Austin shoreline vs Lady Bird • But, aquatic vegetation mitigates shoreline modifications • Non-native vegetation • Drought –lack of flushing • Nutrients C I T Y O F A U S T I N W A T E R S H E D P R O T E C T I O N D E P A R T M E N T 3 Usual Reservoir Updates • Toxic cyanobacteria mats Year 5 of mat monitoring Year 4 of lanthanum-modified bentonite treatments Analyses ongoing • Zebra mussels Dynamic population trends Paper forthcoming • • • • • • Instead, let’s talk plants C I T Y O F A U S T I N W A T E R S H E D P R O T E C T I O N D E P A R T M E N T Aquatic Vegetation! • Submerged (SAV) • Emergent (EAV) • Floating (FAV) https://youtu.be/O2u0lDwm-H8?si=pr191YFXgqWYi1Va C I T Y O F A U S T I N W A T E R S H E D P R O T E C T I O N D E P A R T M E N T Importance • Nutrient uptake • Substrate stability • Wa ter c la rity • Provides habitat, food, refugia of food web components C I T Y O F A U S T I N W A T E R S H E D P R O T E C T I O N D E P A R T M E N T SAV Importance – Case Studies • • I n f l u e n c e o n m a c ro i nve r t e b r a t e s Indicators of ecosystem conditions (e.g., water and habitat quality) Base of food web Analyses ongoing • • • I n f l u e n c e o n l a r g e m o u t h b a s s a n d s e l e c t p r ey s p e c i e s Indicators of ecosystem conditions (e.g., prey and habitat abundance and availability) Published: Bellinger and DeJesus 2024 Southeastern Association of Fisheries and Wildlife Agencies • • C I T Y O F A U S T I N W A T E R S H E D P R O T E C T I O N D E P A R T M E N T Conflicting Perspectives • L a k e A u s t i n • D u r i n g h y d r i l l a p r e s e n c e / m a n a g e m e n t , l a r g e m o u t h b a s s f i s h e r y t h r i v e d 11 legacy-sized (i.e., > 5.9 kg) bass caught between 2010 – 2014 • H o w e v e r, r e c r e a t i o n a l b o a t e r s a n d s w i m m e r s t h o u g h t i t a n u i s a n c e Difficult to power boat Hydrilla associated drowning(?) • M o v e m e n t o f l a r g e r a f t s d a m a g e d To m M i l l e r D a m • • • C I T Y O F A U S T I N W A T E R S H E D P R O T E C T I O N D E P A R T M E N T Vegetation Management • Lake Austin Hydrilla • Managed hydrilla with drawdowns, triploid grass carp (2002 – 2013) • After much discussion between stakeholders C I T Y O F A U S T I N W A T E R S H E D P R O T E C T I O N D E P A R T M E N T SAV boom and bust • Absence of SAV in reser voir • After a decade finally seeing native and non-native plants returning • Boater s (were) loving life • Calls are now coming in to manage plants • Fisherma n (were) hating life • Claimed catching fewer, skinnier Bass Total grass carp SAV C I T Y O F A U S T I N W A T E R S H E D P R O T E C T I O N D E P A R T M E N T 10 Year1998200020022004200620082010201220142016201820202022Triploid Grass Carp stocked (#)0100002000030000400005000060000SAV Coverage (% reservoir)010203040Cummulative #SAV (%) Results – Largemouth Bass • R e l a t i v e w e i g h t ( W r ) ~ f i s h “ p l u m p n e s s ” Bass > 203mm Bass > 356mm • W r-2 0 3 m m • 91.1 ± 1.8 1997 – 2013 2014 – 2022 • 83.1 ± 1.7 • W r-3 5 6 m m • 97.0 ± 2.2 1997 – 2013 2014 – 2022 • 85.6 ± 2.0 Ideal Acceptable • • S i g n i f i c a n t d e c l i n e s i n L M B b o d y c o n d i t i o n a f t e r 2 0 1 3 • Also, a significant monotonic decline through time…. C I T Y O F A U S T I N W A T E R S H E D P R O T E C T I O N D E P A R T M E N T Vegetation on the rebound C I T Y O F A U S T I N W A T E R S H E D P R O T E C T I O N D E P A R T M E N T • Lady Bird Lake • SAV has not been actively managed • • Soft sediments and periodic large flushing events have been effective in “resetting” the system However, during interstitial periods (i.e., low flow droughts), SAV can become prominent and perceptions negative C I T Y O F A U S T I N W A T E R S H E D P R O T E C T I O N D E P A R T M E N T 2013 • Wa t e r ve r y c l e a r a b ove L a m a r water • Enables growth in relatively deep • Positive feedback loop • A c c u m u l a t i o n o f ve g e t a t i o n b e t we e n M o Pa c a n d I - 3 5 n e g a t i ve l y i m p a c t i n g c o n c e s s i o n e r s • • • Evening cruises canceled Aesthetics Collecting trash C I T Y O F A U S T I N W A T E R S H E D P R O T E C T I O N D E P A R T M E N T 2024 So….what do we do? • I n L a ke Au s t i n Non-native growth isolated (for now) to upper reservoir Reservoir operations and functionality are not being impacted = passive WPD management actions at this time • I n L a d y B i r d L a ke Native plant growth wide-spread in reservoir Reservoir operations and functionality are not being impacted Recreation an amenity, not a function, like flood water conveyance Concessioners have been given information/process to hire contractors = passive WPD management actions at this time • • • • • • • • C I T Y O F A U S T I N W A T E R S H E D P R O T E C T I O N D E P A R T M E N T Questions? C I T Y O F A U S T I N W A T E R S H E D P R O T E C T I O N D E P A R T M E N T 16